Confusion caused by the assertion tests

I would like to give some feedback about the tests being run on the most wanted letter example ...

When running the tests, I got a failure on this line of code ... assert checkio("abe") == "a", "The First."

According to your rules about all letters having a count of 1 means we send back the last letter, this assertion fails every time.

assert checkio("One") == "e", "All letter only once."

From: https://py.checkio.org/mission/most-wanted-letter/solve/

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