Baby showers are for when the parents do need all the baby stuff; it's not intended to celebrate the baby. For example, you'd have one for a first baby or if there's been a long period between the youngest and the newest baby. I buy people stuff for subsequent children's births, like clothes or a toy or something. But that's not a baby shower, and I'll do that after the baby's born, whether I attended a shower or not. You don't need to buy a couple a bath set, a baby monitor, a changing table, a crib, a stroller, etc etc etc for every child they have.
In this case, I would probably attend the shower. It wouldn't hurt me to give them a little something, especially since they thought they were done and got rid of all their stuff. I'm sure they didn't sell their old stuff for as much money as they'd need to buy all new stuff.