Steps to reproduce:
1. Create a new Unity project
2. Create new Assembly Definition with name “Test” in Project window
3. Rename Assembly Definition file to “test”
4. Enter Play mode

Expected results:
Nothing unusual happens
Actual results:
Errors appear after renaming the file, preventing you from entering Play mode

Reproduced in: 2018.1.0b1, 2017.3.0p1, 2017.3.0b1

Error: Assembly with name 'Test' already exists (Assets/Test.asmdef)

Note: To fix these errors you can delete the file/move the file to another folder/restart unity

Fixed in: 2018.2.0a5