Count Unique Pairs of Excel Entries Based on Criteria

Here's my problem:

I have two columns. Column A has Customer IDs (e.g., 1, 2, 3, 4, ...), to where I can't possibly list all of the possibilities of ID number. Column B has Product IDs (e.g., A, B, C, D, E), which are a limited set (no more than 15 values).

I need to write a formula that checks:
IF a customer bought product A, did they ALSO buy one of the others (besides another A)?

So far, I've been close with a few options I found elsewhere on the Internet, but I haven't been able to weed out things like if Customer 1 bought Product A twice (which shouldn't be counted).

Also, the data I work with notoriously has blanks/missing data/0 values.

Help??

Try a pivot table, maybe putting Products as a column field, Customer ID as a Row field and Count of Product as data. Alternately, you could put both Customer and Product as Row fields and Count of product as data. See which format you prefer.