It even happened that I played the same melody from a song I had created myself without realizing it.
That is not the issue and it is unavoidable. Musical youtube however is full of "Use these three chords to create great music" recipes. There is just so much stuff that doesn't even try to do anything interesting.
Sounds awfully familiar, like the discussion about synthetic art and generative text. They threaten the professionals with lower barrier to entry, but only take the new comers so far.
Does that detract in any way from the anti-fascist song? Of course not. Folk music has always done this, and that has never taken anything away from its creativity (talking about actual folk music with collective authorship, not commercial music in the "folk" style).
For example parts of the famous Christmas oratorio are originally from a secular cantanta:
https://en.m.wikipedia.org/wiki/Tönet,_ihr_Pauken!_Erschalle...
[0]: https://en.wikipedia.org/wiki/Dance_of_the_Vampires_(musical...