The way it works is, from what i've read from a few sites, the dupe scanner checks to see if another person in the game has it. It's not random for the person to lose the dupe, it's the 2nd person to join.
For example:
If i have a Cliffkiller (which i don't
) and i join (or better yet, make) the game and
Player 2 comes in and he/she too has a cliffkiller ammy.. his/hers disapears because the dupe scanner activates after a person joins a game.
So if you make each game you play, you have 0% chance to lose your ammy. Otherwise, if you join a game and someone that's already there has the ammy, well... i hope you weren't too attached to it emotionally
I'll see if i can find the sources for you to read. I know the one site that had the info is dead, so i'll check around...