How to find and remove the already in use extension

FreePBX
1

trying add ext. 1001 failed due to “already in use: undefined” - how to find and remove the already in use extension, then re-add the extension? (was trying to add but failed, then unable to add any more like above stated)

thanks

2

Reports > Asterisk info > peers.

3

And also
in advanced settings;

Aggresively Check for Duplicate Extensions must be “true”