What project are you trying to complete? Are you trying to make a copy of edited part of the body to clone it and mirror? If yes, then you would need to do weight transfer to another part. When you clone part of the mesh you clone it with all weight, so if you mirror right leg it still has bones that belong to right leg. The only thing that can be done instead of transferring bones is to rename them very carefully changing R to L, of course if cloned part had perfect weights. Otherwise you would need to to weight transfer. Hope I understood you correctly.
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