Add test coverage for failing slider case

osu.Game.Tests/Beatmaps/Formats/LegacyBeatmapDecoderTest.cs

@@ -775,5 +775,22 @@ namespace osu.Game.Tests.Beatmaps.Formats
                 Assert.That(seventh.ControlPoints[4].Type == null);
             }
         }
+
+        [Test]
+        public void TestSliderLengthExtensionEdgeCase()
+        {
+            var decoder = new LegacyBeatmapDecoder { ApplyOffsets = false };
+
+            using (var resStream = TestResources.OpenResource("duplicate-last-position-slider.osu"))
+            using (var stream = new LineBufferedReader(resStream))
+            {
+                var decoded = decoder.Decode(stream);
+
+                var path = ((IHasPath)decoded.HitObjects[0]).Path;
+
+                Assert.That(path.ExpectedDistance.Value, Is.EqualTo(2));
+                Assert.That(path.Distance, Is.EqualTo(1));
+            }
+        }
     }
 }

osu.Game.Tests/Resources/duplicate-last-position-slider.osu

@@ -0,0 +1,19 @@
+osu file format v14
+
+[Difficulty]
+HPDrainRate:7
+CircleSize:10
+OverallDifficulty:9
+ApproachRate:10
+SliderMultiplier:0.4
+SliderTickRate:1
+
+[TimingPoints]
+382,923.076923076923,3,2,1,75,1,0
+382,-1000,3,2,1,75,0,0
+
+[HitObjects]
+
+// Importantly, the last position is specified twice.
+// In this case, osu-stable doesn't extend the slider length even when the "expected" length is higher than the actual.
+261,171,25305,6,0,B|262:171|262:171|262:171,1,2,8|0,0:0|0:0,0:0:0:0: