Drop down list that doesn't allow items selected in other lists.

[Aulakhveer]

I am creating an order form that allows users to select items from a drop down. I want a pop up to show up when they order the same item more than once. I have the following formula for the Data Validation (list) for the drop down
[=INDEX(REFERENCETBL!$A$4:$A$193,AGGREGATE(15,3,(REFERENCETBL!$G$4:$G$193=D22)/(REFERENCETBL!$G$4:$G$193=D22)*(ROW(REFERENCETBL!$G$4:$G$193)-ROW(REFERENCETBL!$G$3)),1))]

and tried including the a countif formula in the Data Validation but i kept getting a Compile Error
[=AND(INDEX(REFERENCETBL!$A$4:$A$193,AGGREGATE(15,3,(REFERENCETBL!$G$4:$G$193=D22)/(REFERENCETBL!$G$4:$G$193=D22)*(ROW(REFERENCETBL!$G$4:$G$193)-ROW(REFERENCETBL!$G$3)),1)),COUNTIF(F22:F50,F22)=1)]

I also tried getting a pop up for duplicate items in VBA but I could get anything to pop up. I feel like it has to do with the Drop list, as most formulas i have seen only get initiated when an item is entered into a cell and not selected from a drop down.

Sorry if I made any errors or if left anything out.