I see composing as starting from a particular point and then branching out in any chosen direction. You then take leaps from interconnected branches and leap between those that are spaced apart, but you ultimately return to the start point. A piece is good and "memorable" if it takes you in surprising directions but still knows how to return to the start.
I agree with a lot at has been said. I think it is kind of Impossible to not plagiarize yourself even just a little. It's why we composers can hear a piece of music and be like, oh that's Mahler, or vaughan Williams etc. of course doing it too much and it's likely people will just not find it very interesting. Honestly that's why I don't like Whitacre that much anymore, i mean his music is absolutely genius but it just seems to lose my attention after awhile. He s just SO distinctive from other composers in that aspect.
That's my view on it anyway
I remember having used ideas from other composers deliberately. Also, unintentional borrowings. I could not care less, really. Composition is not about the source of the material, but how you organize and develop it.
Originality is underrated, anyway. (Originality as in harking back to origins, prototypes... ;-)