Airlines consider duplicate bookings a violation. Their systems are checking bookings against duplicates. When found, airlines might warn and ask to cancel the unnecessary bookings; or they simply cancel everything without a prior notification. Moreover, airlines may even impose a penalty for double bookings.

Duplicate booking definition:

for one and the same person
within one airline or airline group
for the same or different dates

Avoid creating duplicates, as an airline can cancel them without warning and impose penalties.
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