I noticed this also.

This is one aspect where DL is better than other airlines. It marks your duplicate reservations, but does not cancel them. UA would right away cancel all the duplicates. In the post-pandemic and current geopolitical climate, esp. for int'l travel, you often need multiple bookings, as you don't know which flights will operate and what may or may not work as the time comes closer. Kudos to Delta for letting you do that and keep the duplicates. (I've never had dupes within a few days or departure, but I presume at some point closer in DL would void them. But it's of huge help to have duplicate reservations further out, where there's uncertainty or doubts as to exactly when and via what route you'll be able to when.)