This happens a lot when you import models from older video games. I’ve always merged all the parts into one object, delete half the model, apply a mirror modifier, and then remodel the seams as one piece. (Using the mirror modifier means you only have to do one side and it will repeat on the other.
I’ll do this when there is a base “nude” model and there are clothes over it. Merging the clothes with the body will allow you to remove the body modeling under the clothes and save yourself some triangles.
If the seams are properly “hidden” (For example the leg tops are under the skirt you will never see, and not connected to any hip piece) then just leave it. For example, I see the arms are going though sleeve tubes. If the arm is actually inside the sleeve, just delete the arm and leave the shoulder, wrist and hand. Then model the inside of the sleeve “horn” to the wrist, and model the shoulder to sleeve on the other side to close the seam (I’ve had to do this with vocaloid models)
For the shoulder strap, sometimes I’ll apply a Boolean join and do the clean up. *** Warning *** Boolean joins are really messy. Be prepaied to be spending some time fiddling with the topography after the apply. Also a Boolean will not apply if the model is non-manifold (Hollow and closed all around)
Upshot : strip out what’s not appropriate and remodel, you may learn a few things about modeling human anatomy too!