Writing only about the gun:
Kel Tek makes at least 3 models of 9mm semi-auto.
It’s perfectly plausible to have fired 1 round and have “a full magazine” if one was still in the chamber and that counted.
Load magazine, chamber round, remove magazine, add 1 round, reinsert magazine.
This means it is nearly impossible he shot 2X.
When you shoot, the empty casing is ejected and a new round is inserted into place. At that point the mag is minus one round and not full. If one were to fire the weapon and then load one round into the mag then it might appear full.