Count Combinations Across Multiple Columns?

I need to count how many times a code appears over a 6mth period only on a single day. The data would look something like:-

1/1 2/1 3/1 4/1 5/1 6/1 7/1 8/1 9/1 10/1 etc
a b c a a b a a b b

If counting b the result would be 2 on the above 10 days as it appears on the 9/1 & 10/1 it would not be included. It is to be used to count the number of times somebody only appears on a single day over a period of time but not count them if they come back the next day.

I cant think of a way to do this using in built formulas (Countifs, sumproduct, etc) The example is basic and there could be up to 20 codes.