Steps to reproduce:
1. Open a new project
2. Go to Preferences->Keys
3. Find a key and change it to the same as before*

Expected: the key changes
Actual: an error is thrown**

Reproduced in: 2018.2.0a6, 2018.2.14f1, 2018.3.0b8
Not reproduced in: 2017.4.14f1, 2018.1.9f2, 2018.2.0a5, 2019.1.0a7***

Note:
Reproducible on MacOS 10.13
*Sometimes it only reproduces on Keys with dual inputs
**Key [Key] can't be used for action [Action] because it's already used for action [Same Action]
***the shortcut window has been changed completely, but prior versions should still work correctly