Will unchanged database extents result in identical SQL backup bits on disk? What about with backup compression enabled?

(I'm trying to predict long-term SQL backup storage needs and if/where to enable compression, on a storage appliance with de-duplication. I know the best answer is "test both and see", which we are doing.) I seem to recall that SQL backups (and restores) are performed upon SQL database *extents* (8x 8kb data pages). My question is, *are those extents handled predictably and consistently by the SQL backup process* in a way that the storage de-duplication can rely on? By that I mean, if we do a full backup each night, *will the unchanged extents result in identical bits on disk (for those unchanged portions) from night to night?* Or is that not reliable/predictable? What if we enable SQL backup compression? Will identical extents result in identical compressed bits from night to night? Or is that not guaranteed?