Short Answer: They cannot. In the court system they cannot rule on an issue twice - meaning you cannot be foreclosed on twice for the same property.
First I would contact them and send a copy of the final foreclosure judgment/trustee sale document showing them this issue has already been addressed and concluded in court.
If you are still dealing with this issue after that, send them a Debt Validation Letter requesting valid proof of debt. Since they are not the holder of the original note they will not be able to prove valid debt and hence must drop their collection attempts and claim.
If you still are running into issues I suggest speaking with a legal professional.
Best of Luck!