A quick search of the 'net reveals an article written in 1998 said that in order to completely clone a SIM you need physical access to the original. So, unless you've recently misplaced your phone and then had it miraculously reappear, I'd say it's somebody who has trouble reading. Then again, if they've been renting porn videos, maybe their eyesight isn't that good anymore.

I don't know wether two identical phones could also exist on the same network, otherwise, how would the network know which phone to 'ring' when someone dials your number.

In the days of analogue cloning, people only used the ID's to make calls, there would be no point resolving the phone number as no'one would want to incriminate themselves by answering a knowingly cloned phone.