Hmmmm - I've only had one sidelock and never had this problem, but I'm wondering . . . A percussion cap should fit right down over the nipple with only finger pressure. I know there are percussion caps and also Musket Caps, which I think are larger. Perhaps you have a musket cap nipple on there, and the first strike is pushing it down to where the 2nd actually smacks it against some metal to fire it. Someone else will be along soon to either confirm this or give you a better idea.

IM jaybe