I reported your post. 
As Peggy stated, we never seen that happen yet, and I doubt it will as once it is claimed it is marked right away. So an error page would be generated for the other Mod, I would guess (Mike or Kier can you confirm?).
You can unclaim it by changing the status to "Open", if it was not resolved.
If the report was resolved, adding a comment will automatically set the report to Open, and then another Moderator (or even the one who originally resolved it), can claim it.
No way! When I see there is no other staff on, I sneak in, set all claimed reports to open and claim them all for myself.... 
Ok, just kidding, BUT, to do some testing, I did steal one of Mikes, 