Originally Posted by RealHJ

Other airlines (UA, for example) run a sweep and automatically cancel the latest booking(s) that are overlapping any existing flights. Even across continents and timezones. Anything that is impossible.

DL, at least previously, hasn't had such an automated sweep (daily or otherwise). But, it's likely to come up and be flagged for human review. You don't want your account to be locked for audit or what not. If you book multiple flights but only intend to make one (even the same one-way two or more days in a row, not exactly overlapping but quite clearly not feasible to fly all as booked), just let DL know - right away, proactively - why you have it so booked and by when you intend to cancel the duplicate res, and they should notate your PNRs accordingly, so when it gets flagged for manual review there are notes/explanation as to why it's so. That is, if you get a helpful and understanding agent...which is a coin-toss nowadays. AND if you have a legitimate reason for doing so, and it's a one-off and not a pattern of such behavior on the account.