Just a quick note to anyone thinking of modding your skills, so I did some testing the other day. Having three of the same skill equipped does not stack (equipping three Firearm Lv. 3s will not increase your damage by 150%). Having different levels of the same skill equipped does not stack, either (i.e. equipping Firearm Lv 1, 2, and 3 will not increase your damage by 80%).