As mentioned it is possible but very difficult to clone a GSM SIM (so I have read) but they would both be deactivated as soon as they were used at the same time.

I think that either the number has been misprinted in the past or it was allocated to the seller at some time.

Our phone number at work was used by a local turbine impeller manufacturer until 1996ish and we get a few calls a month from people dialing the number printed on old literature.

We know the correct number by heart now and people seem impressed!

Gareth