What i meant was, it is very hard for me to know if a melody was used elsewhere, without me knowing the song. And i don't know how to check if it has already been used. Yes i've heard the melody somewhere, but it's like finding a needle in a haystack. If i made classical music and you told me it's Mozart or something, i couldn't have known. And don't worry i'm not angry.
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I did find out a bit more about this phenomenon, its actually called Cryptomnesia (It is a memory bias whereby a person may falsely recall generating a thought, an idea, a tune, a name, or a joke; they are not deliberately engaging in plagiarism, but are experiencing a memory as if it were a new inspiration.), you can read about it here https://en.wikipedia.org/wiki/Cryptomnesia
And I wasn't joking about the legal stuff, musicians like George Harrison faced legal consequences for subconscious copying, as seen in the "My Sweet Lord" copyright case and had to pay $1,599,987.
I mean in the case of classical music since it is in the public domain you can use it - but it is definitely good practice to credit the composers. What seems to be the issue here is that you used a piece of music in the public domain and passed it on as if it were your original work without crediting the piece or the original composer.
Probably the best way is before you release the work, if you are unsure, you can try asking the community if they can name if the melody of your song appears somewhere else.
I think there are also apps like midomi etc. that let you hum the melody first to check.