Can you show us a close up on the pedal threads, the new ones you bought? It is possible to tell the threading if the image is detailed enough. Will be fairly easy to determine if you have recieved two "left sided" pedals. I think such an image would interest most people in this post, and it can also be the basis for a return claim.
In the OP's defence, aluminium crank threads can strip even if not abused. Especially if the cranks are old, as aluminium does not age gracefully. It may be the case that the original pedals was tough to remove (cheater bar tough). When such forces are involved you can end up stripping the crank thread regardless. Talking from experience.