This code to accomplish the task:
INSERT INTO tblClients ( ContactID, SSN, ClientMailingAddr, City, State, Zip )
SELECT tlbCont.ContactID, TempData.ClientSSN, TempData.ClientMailingAddr1, TempData.ClientMailingAddrCity, TempData.ClientMailingAddrState, TempData.ClientMailingAddrZip
FROM TempData INNER JOIN tblCont AS tlbCont ON (TempData.LastName = tlbCont.LastName) AND (TempData.First = tlbCont.FirstName);
Yes you are so right - as I am identifying missing clients and repeating the process I have the risk of creating duplicate records. When Ichange the join to "OUTER JOIN" however I get "syntax error in FROM clause"