How to reproduce:
1. Open the attached project "Hinges.zip"
2. Open SampleScene
3. Add another Inspector tab and dock it somewhere visible
4. Select 'One' Game Object in the Scene Hierarchy
5. Lock one of the Inspectors
6. Holding CTRL select both 'One' and 'Two' Game Objects
7. Observe the Edit Angular Limits button in the locked Inspector

Expected result: You see one button
Actual result: You see two buttons

Reproducible with: 2019.3.0b12, 2019.4.27f1, 2020.1.0a6, 2020.3.11f1, 2021.1.9f1, 2021.2.0a18
Not reproducible with: 2018.4.34f1, 2019.3.0b11, 2020.1.0a1, 2020.1.0a5

Note:
1. The behavior is different in 2021.2 where one of the Inspectors shows a button and the other one does not display a button at all