Writing a new piece in the language of common practice is almost impossible  without using motives or melodic/rhythmic patterns that others have already invented. Like a child can't learn to speak without using the words it has learnt by it's environment - also we are not able to express ourselves musically without using  the musical elements we have adapted in our learning history, some are conscious and the most are unconscious. The more we are aware of this fact the less we will be disappointed when we realize that what we are "saying" someone other has said this too, it doesn't mean that we have heard it. It is also impossible to have a thought or to google a sentence without finding some references in the web.

So don't bother, this is no question of plagiarism as long it is not expressively planned. It happens all the time to each of us.