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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
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.
The tests are broken, in Python - The expected result and actual result are reversed