highlight if value is duplicate and corresponding cell of all other duplicates is blank

I have a list of clients who contacted us. We need to get back to them.

I would like to format if:

1. They appear twice in the list (got that) and

2. We have not gotten back to them in any of their appearances in the list.

For example: if A5 is duplicated in A478, I want sheets to check cell B478 and format A5 if B478 is blank.