We used to have trouble with the one-eyed cat from next-door coming in, as well as another cat. Changed to a magnetic flap, and all was well (although our cat did loose his magnet once, but we had a spare). But then the one-eyed cat started getting through the cat flap (he didn't have a magnet on his collar) and we couldn't see how he managed it. Then one day I heard the flap moving, next-door's was only lifting the flap up from the outside! After that we kept the flap locked and just let our cat out as and when required.
Wendy