Receive SMS on two SIMS?

Is there anyway that someone could be receving copies of my texts? Either through a cloned SIM or some other method? It seem sthat someone is getting copies of all texts I send or receive. I can't fathom any way of doing it. Unless they had network access of course. Anyone shed any light on how they might be doing such a dubious process?
Any ideas would be greatly appreciated.

Yes, it is possible that your text message which is on the way between two numbers i.e. sender and receiving number, can be receive on other handset in which being use a clone number of your original sim. But not possible to deliver a copy of your text message on both hand sets/number at a time, only one time message will receive here or there.