When duplicating a collapsed blueprint graph which has a node with a custom Exec pin, this results in blueprint compilation error that a duplicate named pin is found. Duplicated exec pins aren't assigned unique names (nor guid, it seems).
User that reported the bug suggested a fix as well, see linked case. I've made a short repro vid too, see attachments.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-161200 in the post.