I had Tom Tom microchipped at the vets when he was first picked up six years ago, he was neutered at the same time, and they scanned him to make sure it was working. two years later when i started microchipping cats myself i scanned him and could not find it, so i microchipped him again, i took him to the vet about a year later to have a xray as he had a bad back and when i went back the vet said i have some bad news tom has two chips, but when i told the vet what had happend he showed me the xray it had moved down one side, so if he ever got lost he has two bless him.