How to reproduce:
1. Create a new Unity project
2. Create multiple grouped Layers (for instance, 'GroupedLayer/Layer1' and 'GroupedLayer/Layer2')
3. Click on the "Layer" button for any GameObject

Expected result: Layers are grouped in a single Layer Group
Actual result: there are multiple Layer Groups with the same name containing only a single Layer

Reproducible with: 2017.2.0a3, 2017.2.3p2, 2017.4.9f1, 2018.1.9f1, 2018.2.3f1, 2018.3.0a8
Not reproducible with: 2017.1.5f1, 2017.2.0a2

Notes:
Grouped Tags do not have this issue