Problem - Minimum Deletions to Make Character Frequencies Unique

Given a string, delete minimum characters so that no two different characters have the same frequency. For $s = "aab"$, frequencies are $a:2$, $b:1$. Already unique, answer is $0$. For $s = "aaabbbcc"$, frequencies are $a:3$, $b:3$, $c:2$. Delete one $a$ or $b$ to make them different. Answer is $1$. Constraint: you can only delete characters, not add them. Deleting reduces a character's frequency. The goal is to make all non-zero frequencies distinct.

If two characters have the same frequency, you must reduce one of them until it differs or reaches zero.