How to reproduce:
1. Open newest tester-submitted project (2Layers1Name.zip)
2. Open "SampleScene.unity" scene
3. Investigate "CollisionTest":

• One sphere is set to layer "Water", the other is set to second "Water" layer instance
• Both cubes are set to "CollisionLayer" layer
• When the scene is played, one sphere collides with cube below, the other falls through(This is intended according to ProjectSettings->Physics->Layer Collision Matrix)

4. Investigate "BounceTest":

• Cube is set to detect bounce of "Water" layer(Notice that LayerMask shows only one instance of "Water" layer)
• Sphere is set to the second instance of "Water" layer
• Play the scene and observe LayerBounceDetection.cs script messages that an unknown layer has hit the cube even though Cube should identify sphere layer as the desired one

5. Investigate "RaycastTest":

• Notice the sphere layer is again set to the second instance of "Water" layer
• Blue raycast("Force of Water") searches for second "Water" layer instance by layer name but instead get the first one and thus does not collide
• Green raycast("Force of Nature") searches for second "Water" layer instance by layer index and is successful - hits the sphere
• Red raycast("Force of Fire") searches for second "Water" layer instance by LayerMask that is defined in inspector but it get the first instance and does not collide too

Expected result: Unity does not let users create a layer with a name that is not unique
Actual result: Unity does let users create a layer with a non-unique name thus introducing inconsistency in physics behaviors

Reproducible with: 2017.1.4p1, 2017.2.3p1, 2017.4.5f1, 2018.1.4f1, 2018.2.0b7