I ordered something online, and then the post office said the package was lost. So the online store shipped me another package, which I received. Then a few weeks after that, I received the "lost" package. What would you do in this scenario?

This has happened a few times to me...I sent the items back. Not saying thats what you should do, thats just what I did. Of course one time they re-sent the item back to me.

send it back after contacting a CSR from the store

Contact customer service for that store. I had this happen a few times. Sometimes you might get lucky and they will let you keep the extra package. (well only got lucky once myself, but better then none)