Duplicate Heads from /gethead Command

A really simple issue. If a moderator/admin was to execute the /gethead command in quick succession, they'd be able to collect the same head multiple times.

I witnessed this second hand when watching a now-demoted moderator spam the command to clear out their claimed heads list on stream.

The issue seems to be the fact that the user can run multiple requests at once and the head it then given before the banned player's name has been marked as already claimed.