honestly, I'm not even sure what my own criteria is for doubles or no xd
Sometimes the items in a collection set will have different color placement for backgrounds, or even layer-ability options depending on the recolor.
There's some things I have doubles of because I like the effects + layering. Like, I can layer multiple poses of same item to achieve a bolder/darker look because using the D skin on the regular has some serious drawbacks.
So for bruises and scrapes, if they work, I pile 'em on. Though there's other things like noses, that this works for too.
Other items, like some of the old EIs and REIs, have such wildly different poses that I like to keep multiples of them.
Sometimes, it's just because the item has a particular shade or tone that exists nowhere else but it and to keep my own aesthetic sanity, I have multiple copies of it.