combo box: changing entrys to avoid duplicates

[carlton123]

i have have a form with a subform which i have a combobox with list of employees. the combo box looks up data from a query that is from a table. the problem is i only want to select the same employee once per subform ie every record in main for can only have joe bloggs once in the subform. is there any way to take out selections from combo box so i cant select same employee twice


thanks

rich