Passenger door dead locking problem
 

I have a problem with my MX5 Mk3 which has me foxed.
I have just bought this 2012 car secondhand.
The passenger door has been set in such a way that it always has to be unlocked twice.
The first unlock allows the door to open about 15mm, then the key fob must be pressed again.
I can remove the feature by using the double locking option from the manual (lock twice within 3 seconds) and then unlocking. After that the passenger door responds normally.
But leave the car for about 10 minutes and the dead-locking feature re-establishes itself.
Its really annoying and nothing in the manual says anything about it.
Any idea how to get rid of this and revert to normal?