Field ID clash

[David V Glasgow]

Using 9.6.0 on Mac 10.13.6

I duplicated a field and found a third field responded to the mouseup of the duplicate.  It seems the duplication generated a clash of IDs between the duplicate and third field.  I could just select and start again, but I was wondering whether there is a safe way to manually (or force automatic) change the ID of one pr other of the fields.  I also wondered how this might happen in the first place.  Is my stack OK, or might a more serious problem be developing? 

Any thoughts or suggestions?

Best wishes,

David Glasgow