Easy. You don’t make duplicate or impossible for the same day(s). They’re generous with speculative bookings in the sense of “I want to go to Paris next month but I may not go” but not “my vacation is next month and I’m going to book Paris and Tokyo simultaneously and decide later.”

Every carrier does this. Some are just better about enforcing it than others. I sort of recall AA logic canceling the older of the two duplicates, and keeping the newer one.

The written policy is in the contract of carriage, but not the details of the algo for batch deletion. In my experience, if you can fly both, it will generally leave you alone. There’s a long boring thread about this somewhere else on Flyertalk with data points if you want to see what you’re likely to be able to get away with.