Is there a quick way to find which items appear in both of two collections?

I have a collection of all items I want to look at (a longlist of 450 items), and two additional collections which I have separated the long list into (shortlist and not-keeping). Each item from the longlist should only appear in one of the additional collections, but the total item count for shortlist and not-keeping combined is 451. I must have put one item in both of the additional collections or have a duplicate item in one collection, but despite going through them manually I can't find the extra item. Is there a way to do this that doesn't rely on my fallible, tired eyes?

Thanks very much for any help anyone can give me.