Count Cells that have more than 1 unique combination

Hi All

I have a simple spreadsheet with 2 columns

What i want do is to somehow flag out rows where the concatenate of column A and B (column C) has 2 or more unique values

As you can see from my example

Item A is disregarded since there is only 1 possible such combination
Item B is flagged because it has more than 1

How do I do this? I'd prefer not to use any advanced autofilter

Please try at D1 Press Ctrl+Shift+Enter and drag down

=SUM(--(FREQUENCY(IF($A$1:$A$90=A1,MATCH($B$1:$B$90,$B$1:$B$90,)),ROW($B$1:$B$90))>0))>1

Please attached your sheet, Go Advanced > Manage Attachments