Is there a way in which I can count cells from particular columns which
contain certain strings but to not count duplicates?

Example

A B
1 Patient number Status
2 1234 Unwell
3 5678 Well
4 9876 Well
5 1234 Unwell
6 9234 Well
7 3456 Unwell

In the above example I want to count the number of unwell patients. However
one of the patients (number 1234 in column A) is duplicated. Therefore, the
number of unwell patients is actually 2, not 3 as would be the answer if I
simply counted the number of unwell entries in column B

Thanks