Format cells with duplicate values but only for visible rows in a filtered list

[glad_ir]

Hi,

I wonder if somebody can help me with this one please.

I have a sheet with approximately 10000 rows of names and addresses. The data is filtered on several columns to hide rows that aren't of interest leaving approximately 3000 rows visible. One of the columns contains postcodes. I would like the sheet to colour cells containing duplicate postcodes but only duplicates wrt the 3000 visible rows. I have tried conditional formatting, but this also considers hidden rows when looking for duplicates. Is there a way to look for duplicates just for visible rows?

Any help would be much appreciated!

thanks,
Iain