It is not too difficult to write a tune that, in all good faith, sounds a bit or even more than a bit, like another. We all absorb tunes and phrases of them that we hear, and they may sink into the subconscious only to re-emerge when we try to write a "new" tune. As has been said before, there are only so many notes, and therefore so many potential combinations and sequences of them. (Actually any statistician might challenge that!)
If I am writing a new tune myself, I will usually run it past a couple of knowledgeable friends to see if they detect any elements of accidental copying of other tunes, and if so, will re-work the tune to eliminate that.
One of my friends came up with a new song, which, to me, sounded as if he had copied the tune from another "in copyright" song. I spoke to him privately of my suspicions and sang him the other song. He acknowledged how similar it was, but could not recollect ever having heard the other song. But he did change the tune of his own song.