In Mafia Wars in Facebook if you have 2 of the same weapon does the attack and defense stats stack and if so why dont people just buy 100 colt pistols

Each of yer mafia members can wield one weapon, one armor and use one vehicle.
the added sum of all the members of you mafia with your equipment will be the one that's compared to the opponents stats on the same field, to calculate who wins a fight

So simply; no, weapons don't stack more then the amount of mafia members you have; so getting 100 knives will only be good if you have 100 mafia members, tho then you might aswell get a better weapons since the total stats is the importance.