Currently you can search for questions closed as duplicate by using a search option
duplicate:1. What if to interpret this number as a number of origins? 0 and 1 are already working as expected, 2 might be used for questions with two or more targets, and so on...
Maybe there’s another way (by SEDE) to find such questions, but SEDE is slightly more hard to use for regular user (and works with outdated data). I think questions with multiple origins could be considered as suspicious:
- Either it may covers multiple problems, and should be closed as too-broad
- or the origins should be closed as duplicate of another one from the list (another case - they need to be improved).