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Yes you can. You can buy multiple tickets to the same location on different day with travel dates prior to a previously purchased ticket.
For example you can purchase an American Airline (or other airline) ticket to London leaving on 10 March and returning on 17 March and immediately after that purchase the same itinerary on the same or even different airline leaving tomorrow February 26 and returning any day in the future.
The fact that you already have a ticket to the destination on a later date does not preclude you from buying another earlier ticket to the same destination.
I have previously done that for routes I frequently travel.
What you canβt do though, is use a return leg without taking the outbound flight.