If you install multiple decal/face packs that contain the same files then it there will be multiple copies in the avatar menu.
However, when you connect to a server and it tells you what decals people are using, your client will first look for that exact file in that exact decal pack. If it doesn't find that, it will fall back to looking for a match for the file name only (in any decal pack). This means that it's possible for playerA to be using a bunch of separate packs and playerB to be using one giant "all in one" pack and they'll sync up.
It also means that it's a good idea to name your face/decal files intelligently dont just have a bunch of things called face1, face2, etc