As others have said, take it to the vet to see if it has an identity chip. I'm afraid your other cat will probably have fleas by now poppy, caught from the stray. You will need to get something like Advocate and Stronghold from your vet. I agree with daffy about Frontline and flea collars. Once the fleas/eggs are in your home, ie on cushions, chairs, sofas, carpets etc you will need something like RIP to get rid of them. If the cat has been given flea treatment by somebody else then it clearly hasn't worked and you will need to have it treated quickly. Bless you for taking it in.