CountIfs(?) Ignoring Duplicates

Hello,

I'm trying to come up with a formula where I can count the number of unique individuals overdue on an action, but not have them counted twice. If they are overdue on 1 action it equals 1, if they are overdue on 50 actions it equals 1. Some individuals may not be overdue at all. So I need a single cell that can evaluate a range and determine total unique individuals overdue.

For example:

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Although 3 actions are overdue (<0), only 2 unique people are overdue so the formula would return 2.

Anyone have a solution for something similar they've tackled? Thank you!

=SUM(--(MATCH((B2:B10=B2:B10)*(C2:C10<0),(B2:B10=B2:B10)*(C2:C10<0),)=ROW(A1:A9))) as array formula

Both of those worked! Now I'll spend some dissecting why/how they worked. Thanks so much both of you!