I suppose you can calculate which ones are removed by dedup, just not by using dedup.
| eventstats last(curious) as remaining by deduping
| stats values(curious) as all by deduping remaining
| eval removed = mvmap(all, if(all == remaining, null(), all))
Here, curious is the field you want to examine, and deduping is the field on which to dedup.
Use the keepevents option to retain the duplicate events, but without the repeated field value. Look for where the field is set to null.