I see my mistake, repeated once means each letter appears twice. The kata is okay

The instruction is misleading - It states: Given a string, you have to return a string in which each character (case-sensitive) is repeated once.
"return a string in which each character (case-sensitive) is repeated once" should be changed to "return a string in which each character (case-sensitive) is repeated twice". That would help.

Not a kata issue. Your code is wrong, you are removing duplicates in the string whilst the question asks to double each character in the string.

Also, you are misunderstanding the logs

'String' should equal 'SSttrriinngg'

Meaning that

actual (what your code returned) should equal Expected correct result from the tests

The tests are broken, in Python - The expected result and actual result are reversed