Can't figure out duplicates plugin tiebreak

[xeruf]

I wanted to tiebreak based on track number - so that singles are preferably eliminated (which by default have a tracknumber of 1). But adding this does not change my results:

duplicate:
    tiebreak:
    - items: [track, bitrate]

Is there another way to accomplish this? I want to eliminate all singles in my library which are also contained in albums.