It says "Someone has already added that SSH key to another account."
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This happens when you try to use the same key for two different accounts. You should not be doing this, a key should belong to a single (user) account.
There's no easy way to find out which of your other accounts may have already uploaded the key, and hence no way to remove it.
The solution, assuming you don't want to use the older account, or can't find out where it is, is to generate a new key for the newer account and add that instead.
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