Field Calc - First two characters of a string check

I have a large dataset with field named 'MAP_NUMBER' in which should contain values of A1-A29, B1-B29 and so forth... Many of the records contain values similar to 'YY29' and should just be 'Y29' . How can I check the second character against the first (which should never be duplicate) and make corrections in the field calculator using python?

Thanks in advance.