For an example, check out this search:
You'll notice that there are tons of hits on this. I've scanned through the list and found a few dupes, but I'm sure there are more.
It would be easier to scan user lists like this if we had client-side sorting. Being able to sort on each column (name, email, id) would make it much easier to find duplicates in situations like this one, where a user search results in tons of hits.