It has been said that there are probably no longer any truly original musical phrases left for us to lay claim to which have not been used in one form or another umpteen times before. It is all a matter of tempo, key, and the combinations in which those phrases are re-gurgitated over and over again which gives rise to a new melody. (Feel free to tell me if I am talking nonsense.)
There is always this problem when you write tunes and is why I post mine here.After playing them through a few times they start to sound so familiar that you wonder why nobody's thought of it before.It is one of the benefits of playing for years to recognise that part, or all of what you are writing is a revamped version of what you have written before, or pinched from another tune. It is not the point to give up, but a slight change of direction.
Depends on how long a phrase, I would say.
So I was very surprised recently when an Australian musical group was required to pay (many years of) royalties to the owner of the copyright (the current owner is not
the composer) of the song "Kookaburra", because they used a 2-measure
excerpt of the tune as part of a flute solo on a bridge in their own hit. And they used it because the song has become a symbol of Australian folk culture,and nobody seemed to be aware that it had a known composer. (I'm trying to remember all the collections -- including publications for American schools -- where I've seen the entire song, and whether any of them listed a composer.) Frankly, I'd be surprised if those two bars hadn't been previously used by someone like Bach, Beethoven, or Mozart, which I believe should have nullified the copyright argument.
One technique I like to use when composing tunes is to start with the first few notes of a familiar tune, but then continue it in a different direction, a direction which I hope others haven't used before me.