How to reproduce:
1. Open any project with Cinemachine installed
2. Add a "Cinemachine Camera" Component to a GameObject
3. In the Component, select an Extention in the "Add Extention" field 2 times
Actual result: A Component is added to this GameObject after a single click, after the second one, the field for "Add Extention" is filled with the Component the user is trying to add again
Expected result: After the second time the user is adding a Component, some indication should be present that the Component is already added, a Warning sign next to the field to indicate this perhaps
Reproducible with: 3.0.0-pre.7