Microchips do not have to be replaced as long as they can be scanned and read. If a scanner is unable to read the chip, check the batteries, as some people do not realise they can run down.
If a dog is chipped more than once, the scanner will read the chip that it is nearest to, but will then read the other one if it is moved over it.
It has been known for chips to move so the scanner should be passed over the entire dog to check. Most chips these days should not move, as their design makes them more able to stay in place.
If you take in a dog or buy one and you don't know the full details get your vet to check it to see if it has been microchipped as it may have been lost or stolen and the chip could reveal a previous owner's details.