Countif data validation

I am using countif(table1[Part Number], b3) <2

How to do this tables as some tables will have multiple instances and some tables unique only.

I think it looks as all tables instead of the table I am referencing.

You may need a better explanation of your problem.

Say i have 1 table that u have unique parts in
And then a table with parts received so the same part can be in the column more than once.both have a column part number

If i ise the formula in data validation for unique values it wontlet me have the same part number in the other table. Say the parts received table i cannot have a part number thta was in the parts table.

Countif([part number],[part number])<2

Can you attach a sample of the file showing what the problem is?