How is it possible that someone cloned my Youtube account, any idea what software they used for cloning?

As far as I know when the username already exist it will tell you that it's already taken. Please help and let me know how to prevent this from happening in the future. Thanks.
How is it possible that someone cloned my Youtube account, any idea what software they used for cloning?
A username can only be used once. However, the user name JOHNDOE johndoe JohnDoe are considered different user names
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