Two things I notice:
1) You applied a second Server Validations server behavior to add the unique key validation... you should have added it to the existing one.
2) When you filter your recordset you are using: $_POST['dbmSiteID'] but when you insert you are using: $_POST["dbmDisplaySiteID"] (there is a similar discrepancy with the Screen field). These need to be the same so that you are looking up the same values that you are inserting.