“Cryptomnesia occurs when a forgotten memory returns without its being recognized as such by the subject, who believes it is something new and original. It is a memory bias whereby a person may falsely recall generating a thought, an idea, a tune, a name, or a joke,[1] not deliberately engaging in plagiarism but rather experiencing a memory as if it were a new inspiration.”
It’s quite common:
“Cryptomnesia occurs when a forgotten memory returns without its being recognized as such by the subject, who believes it is something new and original. It is a memory bias whereby a person may falsely recall generating a thought, an idea, a tune, a name, or a joke,[1] not deliberately engaging in plagiarism but rather experiencing a memory as if it were a new inspiration.”
https://en.wikipedia.org/wiki/Cryptomnesia