My first guess would be the common broken wire in the rubber boot to the door. The alarm still thinks the door is open so goes off. If my alarm goes accidently (forgot son in car is the norm!) off it usually takes several goes with the remote to stop it. Dont know why buy its performs fine every other way so Ive never bothered to investigate.
Pull back the rubber boot and check those wires.....just a guess but not a lot else to go on without seeing the car.
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