Jimmy's first statement that he is a knave can be made by all types but a knight.
Johnny said Jimmy is not a knight,so Johnny cannot be a liar.
Jimmy can either be a knave who told the truth first and lied second or a liar who always lies.
Either way,his second statement is a lie,so Johnny is not a knave.
That proves Johnny is a knight.
Therefore,his second statement was true,so Jimmy is a knave.
In summary,Jimmy is a knave and Johnny is a knight.
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