Sorry buddy, but there is such a thing as plagiarism. And, despite the fact that it may happen that you inadvertently plug a section into one of your songs that you actually have heard before, it is also your responsibility as an artist to mitigate the effect. Change the arrangement, whatever.
And just to once and for all get rid of this pseudo-mathematical argument "there's only so many notes, you're bound to run into this":
In a single 4/4 bar of music, the probability of creating the same sequence of notes is:
(1/2* 1/96) ^ 16
1/2 from note on or off
1/96 from 4 octaves of notes to choose from
0.000000000000000000000000000000000029%
=> 2.9*10^-37
And that doesn't include choice of instrument, tone, multiple notes etc. etc.
Yup. In a single bar of music, your chance of accidentally hitting the same sequence of notes is about the same as playing the lottery which planet of the universe to land on, and then landing on Earth.
rumborak