diff --git a/osu-framework b/osu-framework
index f27e36d405..14eb531c00 160000
--- a/osu-framework
+++ b/osu-framework
@@ -1 +1 @@
-Subproject commit f27e36d405dd3f041e19defd59ecbb389ba84617
+Subproject commit 14eb531c0056b8569f21b3571890383ffbea768e
diff --git a/osu.Game/Beatmaps/BeatmapManager.cs b/osu.Game/Beatmaps/BeatmapManager.cs
index ff0abd3d78..006269f186 100644
--- a/osu.Game/Beatmaps/BeatmapManager.cs
+++ b/osu.Game/Beatmaps/BeatmapManager.cs
@@ -483,7 +483,8 @@ namespace osu.Game.Beatmaps
             using (var stream = new StreamReader(reader.GetStream(mapName)))
                 metadata = BeatmapDecoder.GetDecoder(stream).Decode(stream).Metadata;
 
-            beatmapSet = new BeatmapSetInfo
+            // check if a set already exists with the same online id.
+            beatmapSet = beatmaps.BeatmapSets.FirstOrDefault(b => b.OnlineBeatmapSetID == metadata.OnlineBeatmapSetID) ?? new BeatmapSetInfo
             {
                 OnlineBeatmapSetID = metadata.OnlineBeatmapSetID,
                 Beatmaps = new List<BeatmapInfo>(),
@@ -510,16 +511,21 @@ namespace osu.Game.Beatmaps
                     beatmap.BeatmapInfo.Hash = ms.ComputeSHA2Hash();
                     beatmap.BeatmapInfo.MD5Hash = ms.ComputeMD5Hash();
 
-                    // TODO: Diff beatmap metadata with set metadata and leave it here if necessary
-                    beatmap.BeatmapInfo.Metadata = null;
+                    var existing = beatmaps.Beatmaps.FirstOrDefault(b => b.Hash == beatmap.BeatmapInfo.Hash || b.OnlineBeatmapID == beatmap.BeatmapInfo.OnlineBeatmapID);
 
-                    RulesetInfo ruleset = rulesets.GetRuleset(beatmap.BeatmapInfo.RulesetID);
+                    if (existing == null)
+                    {
+                        // TODO: Diff beatmap metadata with set metadata and leave it here if necessary
+                        beatmap.BeatmapInfo.Metadata = null;
 
-                    // TODO: this should be done in a better place once we actually need to dynamically update it.
-                    beatmap.BeatmapInfo.Ruleset = ruleset;
-                    beatmap.BeatmapInfo.StarDifficulty = ruleset?.CreateInstance()?.CreateDifficultyCalculator(beatmap).Calculate() ?? 0;
+                        RulesetInfo ruleset = rulesets.GetRuleset(beatmap.BeatmapInfo.RulesetID);
 
-                    beatmapSet.Beatmaps.Add(beatmap.BeatmapInfo);
+                        // TODO: this should be done in a better place once we actually need to dynamically update it.
+                        beatmap.BeatmapInfo.Ruleset = ruleset;
+                        beatmap.BeatmapInfo.StarDifficulty = ruleset?.CreateInstance()?.CreateDifficultyCalculator(beatmap).Calculate() ?? 0;
+
+                        beatmapSet.Beatmaps.Add(beatmap.BeatmapInfo);
+                    }
                 }
             }
 
diff --git a/osu.Game/Database/OsuDbContext.cs b/osu.Game/Database/OsuDbContext.cs
index 928c355696..9c1413b93b 100644
--- a/osu.Game/Database/OsuDbContext.cs
+++ b/osu.Game/Database/OsuDbContext.cs
@@ -78,6 +78,7 @@ namespace osu.Game.Database
         {
             base.OnModelCreating(modelBuilder);
 
+            modelBuilder.Entity<BeatmapInfo>().HasIndex(b => b.OnlineBeatmapID).IsUnique();
             modelBuilder.Entity<BeatmapInfo>().HasIndex(b => b.MD5Hash).IsUnique();
             modelBuilder.Entity<BeatmapInfo>().HasIndex(b => b.Hash).IsUnique();
 
diff --git a/osu.Game/Migrations/20171119065731_AddBeatmapOnlineIDUniqueConstraint.Designer.cs b/osu.Game/Migrations/20171119065731_AddBeatmapOnlineIDUniqueConstraint.Designer.cs
new file mode 100644
index 0000000000..b2f81a729a
--- /dev/null
+++ b/osu.Game/Migrations/20171119065731_AddBeatmapOnlineIDUniqueConstraint.Designer.cs
@@ -0,0 +1,302 @@
+// <auto-generated />
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage;
+using osu.Game.Database;
+using System;
+
+namespace osu.Game.Migrations
+{
+    [DbContext(typeof(OsuDbContext))]
+    [Migration("20171119065731_AddBeatmapOnlineIDUniqueConstraint")]
+    partial class AddBeatmapOnlineIDUniqueConstraint
+    {
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder
+                .HasAnnotation("ProductVersion", "2.0.0-rtm-26452");
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapDifficulty", b =>
+                {
+                    b.Property<int>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<float>("ApproachRate");
+
+                    b.Property<float>("CircleSize");
+
+                    b.Property<float>("DrainRate");
+
+                    b.Property<float>("OverallDifficulty");
+
+                    b.Property<float>("SliderMultiplier");
+
+                    b.Property<float>("SliderTickRate");
+
+                    b.HasKey("ID");
+
+                    b.ToTable("BeatmapDifficulty");
+                });
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapInfo", b =>
+                {
+                    b.Property<int>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<int>("AudioLeadIn");
+
+                    b.Property<int>("BaseDifficultyID");
+
+                    b.Property<int>("BeatDivisor");
+
+                    b.Property<int>("BeatmapSetInfoID");
+
+                    b.Property<bool>("Countdown");
+
+                    b.Property<double>("DistanceSpacing");
+
+                    b.Property<int>("GridSize");
+
+                    b.Property<string>("Hash");
+
+                    b.Property<bool>("Hidden");
+
+                    b.Property<bool>("LetterboxInBreaks");
+
+                    b.Property<string>("MD5Hash");
+
+                    b.Property<int?>("MetadataID");
+
+                    b.Property<int?>("OnlineBeatmapID");
+
+                    b.Property<string>("Path");
+
+                    b.Property<int>("RulesetID");
+
+                    b.Property<bool>("SpecialStyle");
+
+                    b.Property<float>("StackLeniency");
+
+                    b.Property<double>("StarDifficulty");
+
+                    b.Property<string>("StoredBookmarks");
+
+                    b.Property<double>("TimelineZoom");
+
+                    b.Property<string>("Version");
+
+                    b.Property<bool>("WidescreenStoryboard");
+
+                    b.HasKey("ID");
+
+                    b.HasIndex("BaseDifficultyID");
+
+                    b.HasIndex("BeatmapSetInfoID");
+
+                    b.HasIndex("Hash")
+                        .IsUnique();
+
+                    b.HasIndex("MD5Hash")
+                        .IsUnique();
+
+                    b.HasIndex("MetadataID");
+
+                    b.HasIndex("OnlineBeatmapID")
+                        .IsUnique();
+
+                    b.HasIndex("RulesetID");
+
+                    b.ToTable("BeatmapInfo");
+                });
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapMetadata", b =>
+                {
+                    b.Property<int>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<string>("Artist");
+
+                    b.Property<string>("ArtistUnicode");
+
+                    b.Property<string>("AudioFile");
+
+                    b.Property<string>("AuthorString")
+                        .HasColumnName("Author");
+
+                    b.Property<string>("BackgroundFile");
+
+                    b.Property<int>("PreviewTime");
+
+                    b.Property<string>("Source");
+
+                    b.Property<string>("Tags");
+
+                    b.Property<string>("Title");
+
+                    b.Property<string>("TitleUnicode");
+
+                    b.HasKey("ID");
+
+                    b.ToTable("BeatmapMetadata");
+                });
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapSetFileInfo", b =>
+                {
+                    b.Property<int>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<int>("BeatmapSetInfoID");
+
+                    b.Property<int>("FileInfoID");
+
+                    b.Property<string>("Filename")
+                        .IsRequired();
+
+                    b.HasKey("ID");
+
+                    b.HasIndex("BeatmapSetInfoID");
+
+                    b.HasIndex("FileInfoID");
+
+                    b.ToTable("BeatmapSetFileInfo");
+                });
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapSetInfo", b =>
+                {
+                    b.Property<int>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<bool>("DeletePending");
+
+                    b.Property<string>("Hash");
+
+                    b.Property<int?>("MetadataID");
+
+                    b.Property<int?>("OnlineBeatmapSetID");
+
+                    b.Property<bool>("Protected");
+
+                    b.HasKey("ID");
+
+                    b.HasIndex("DeletePending");
+
+                    b.HasIndex("Hash")
+                        .IsUnique();
+
+                    b.HasIndex("MetadataID");
+
+                    b.HasIndex("OnlineBeatmapSetID")
+                        .IsUnique();
+
+                    b.ToTable("BeatmapSetInfo");
+                });
+
+            modelBuilder.Entity("osu.Game.Input.Bindings.DatabasedKeyBinding", b =>
+                {
+                    b.Property<int>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<int>("IntAction")
+                        .HasColumnName("Action");
+
+                    b.Property<string>("KeysString")
+                        .HasColumnName("Keys");
+
+                    b.Property<int?>("RulesetID");
+
+                    b.Property<int?>("Variant");
+
+                    b.HasKey("ID");
+
+                    b.HasIndex("IntAction");
+
+                    b.HasIndex("Variant");
+
+                    b.ToTable("KeyBinding");
+                });
+
+            modelBuilder.Entity("osu.Game.IO.FileInfo", b =>
+                {
+                    b.Property<int>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<string>("Hash");
+
+                    b.Property<int>("ReferenceCount");
+
+                    b.HasKey("ID");
+
+                    b.HasIndex("Hash")
+                        .IsUnique();
+
+                    b.HasIndex("ReferenceCount");
+
+                    b.ToTable("FileInfo");
+                });
+
+            modelBuilder.Entity("osu.Game.Rulesets.RulesetInfo", b =>
+                {
+                    b.Property<int?>("ID")
+                        .ValueGeneratedOnAdd();
+
+                    b.Property<bool>("Available");
+
+                    b.Property<string>("InstantiationInfo");
+
+                    b.Property<string>("Name");
+
+                    b.HasKey("ID");
+
+                    b.HasIndex("Available");
+
+                    b.ToTable("RulesetInfo");
+                });
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapInfo", b =>
+                {
+                    b.HasOne("osu.Game.Beatmaps.BeatmapDifficulty", "BaseDifficulty")
+                        .WithMany()
+                        .HasForeignKey("BaseDifficultyID")
+                        .OnDelete(DeleteBehavior.Cascade);
+
+                    b.HasOne("osu.Game.Beatmaps.BeatmapSetInfo", "BeatmapSet")
+                        .WithMany("Beatmaps")
+                        .HasForeignKey("BeatmapSetInfoID")
+                        .OnDelete(DeleteBehavior.Cascade);
+
+                    b.HasOne("osu.Game.Beatmaps.BeatmapMetadata", "Metadata")
+                        .WithMany("Beatmaps")
+                        .HasForeignKey("MetadataID");
+
+                    b.HasOne("osu.Game.Rulesets.RulesetInfo", "Ruleset")
+                        .WithMany()
+                        .HasForeignKey("RulesetID")
+                        .OnDelete(DeleteBehavior.Cascade);
+                });
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapSetFileInfo", b =>
+                {
+                    b.HasOne("osu.Game.Beatmaps.BeatmapSetInfo")
+                        .WithMany("Files")
+                        .HasForeignKey("BeatmapSetInfoID")
+                        .OnDelete(DeleteBehavior.Cascade);
+
+                    b.HasOne("osu.Game.IO.FileInfo", "FileInfo")
+                        .WithMany()
+                        .HasForeignKey("FileInfoID")
+                        .OnDelete(DeleteBehavior.Cascade);
+                });
+
+            modelBuilder.Entity("osu.Game.Beatmaps.BeatmapSetInfo", b =>
+                {
+                    b.HasOne("osu.Game.Beatmaps.BeatmapMetadata", "Metadata")
+                        .WithMany("BeatmapSets")
+                        .HasForeignKey("MetadataID");
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}
diff --git a/osu.Game/Migrations/20171119065731_AddBeatmapOnlineIDUniqueConstraint.cs b/osu.Game/Migrations/20171119065731_AddBeatmapOnlineIDUniqueConstraint.cs
new file mode 100644
index 0000000000..d3830ec0f5
--- /dev/null
+++ b/osu.Game/Migrations/20171119065731_AddBeatmapOnlineIDUniqueConstraint.cs
@@ -0,0 +1,25 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+using System;
+using System.Collections.Generic;
+
+namespace osu.Game.Migrations
+{
+    public partial class AddBeatmapOnlineIDUniqueConstraint : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateIndex(
+                name: "IX_BeatmapInfo_OnlineBeatmapID",
+                table: "BeatmapInfo",
+                column: "OnlineBeatmapID",
+                unique: true);
+        }
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_BeatmapInfo_OnlineBeatmapID",
+                table: "BeatmapInfo");
+        }
+    }
+}
diff --git a/osu.Game/Migrations/OsuDbContextModelSnapshot.cs b/osu.Game/Migrations/OsuDbContextModelSnapshot.cs
index 7029dcdcd5..e3f1cf798b 100644
--- a/osu.Game/Migrations/OsuDbContextModelSnapshot.cs
+++ b/osu.Game/Migrations/OsuDbContextModelSnapshot.cs
@@ -103,6 +103,9 @@ namespace osu.Game.Migrations
 
                     b.HasIndex("MetadataID");
 
+                    b.HasIndex("OnlineBeatmapID")
+                        .IsUnique();
+
                     b.HasIndex("RulesetID");
 
                     b.ToTable("BeatmapInfo");
diff --git a/osu.Game/osu.Game.csproj b/osu.Game/osu.Game.csproj
index 7b479bdba2..3b4434d1f4 100644
--- a/osu.Game/osu.Game.csproj
+++ b/osu.Game/osu.Game.csproj
@@ -279,6 +279,10 @@
     <Compile Include="Migrations\20171025071459_AddMissingIndexRules.Designer.cs">
       <DependentUpon>20171025071459_AddMissingIndexRules.cs</DependentUpon>
     </Compile>
+    <Compile Include="Migrations\20171119065731_AddBeatmapOnlineIDUniqueConstraint.cs" />
+    <Compile Include="Migrations\20171119065731_AddBeatmapOnlineIDUniqueConstraint.designer.cs">
+      <DependentUpon>20171119065731_AddBeatmapOnlineIDUniqueConstraint.cs</DependentUpon>
+    </Compile>
     <Compile Include="Migrations\OsuDbContextModelSnapshot.cs" />
     <Compile Include="Online\API\Requests\GetBeatmapSetRequest.cs" />
     <Compile Include="Online\API\Requests\GetBeatmapSetsResponse.cs" />