Match positive and negative values with the same assignment/reference

Hi all,

I want to match (highlight cells) positive and negative values per assignment. So in column A you have the assignment (ex. 1, 2, 3, 4 and 5), and in column B you have the values. So if the rows have the same assignment and have the values -5000 and 5000 they should get highlighted. But I cannot figure out any way to make this work properly with conditional formatting. But if there are two rows with the same assignment with the values of -5000 and 4000 they should not be highlighted. I have attached an example.
example.xlsx

Many thanks for your help, I hope this problem is possible to solve!

Best Regards,
Nicklas