Since so many schticks specify exactly what benefits you get when you purchase them multiple times, and also specifically mention the limits on multi-purchases for that schtick, I'd rule that since the Auxiliary Schticks don't include that information, they can only be purchased once, for the stated benefits.
But for the schticks that specify the advantage of multiple purchase, but not that the limit is on multi-purchase when schtick-picking, is there a rule as to a limit? I'm tempted by a "Maximum 5" rule off-hand, but I might have missed a sentence in the rule book (which is currently in the wrong city, following a packing error, and with which I'll be reunited in a while, but that's by-the-by).
I tend to think of 5 as a reasonable maximum, but by the rules a schtick that says you can buy multiples and doesn't explicitly limit a certain number essentially has no max. For gun schticks, I think most of them were deliberately designed that way because of the limited number of them.
If a player wants to push beyond 5, then I let him know he's essentially telling me as a GM that he wants to get into an Whoop-Ass Escalation War, which is fine, but he needs to know that I've got unlimited ammo, and anything he can break I can break even worse.