No duplicate entries in Lookup Field

No duplicate entries in Lookup Field

I have two Forms: FormA and FormB.
I have a Lookup in FormB called "LastName" wich imports data from "LastName" in FormA (dropdown single select).
"LastName" in FormA is not able to have duplicate entries.
Is there anyway to avoid having "LastName" in FormB with duplicate entries?
In other words, once one LastName has been selected in FormB, it won�t appear in that dropdown list for the next records.

Thank you very much for your software.