To start, I have requested an administrator to have this thread moved as it is supposed to be in the Help Section rather than Host Support.
Since the version 2.2.2 R4 revision, players will only be able to own one of each weapon from Ammunation when purchasing legal weapons. So for example, you could buy a Desert Eagle from Ammunation but would not be able to buy another one until the first weapon is destroyed regardless of whether or not you have the weapon in your possession.
However I really couldn't provide an answer regarding the occurrence of situations like being robbed. I'll leave this to TommyB.
The script was implemented in order to prevent players from abusing weapons licenses by purchasing weapons and then selling them to their friends. Once the weapon is removed from the database by any means (whether by /disarm from an admin or /take from the SASP, etc) then you'll be able to buy a new one.