But that's not exactly what I look for.
Your function for, e.g. Aaa, Xxx, Aaa, Xxx, Aaa, Yyy, Bbb, Xxx shows "3"
although it should give "2", because we have just 2 pairs of Aaa and Xxx.

=SUMPRODUCT(EXACT(A1:A7,"Aaa")*EXACT(A2:A8,"Xxx"))

assuming you want case-sensitive string comparisons.

Great! I've just made several tests and everything seems to work perfectly
so far!
Thank you very much for your help, Harlan. Thank you, everyone.
Dziekuje!

--
Piotrek