I haven't done any serious analysis of Time Machine (or of APFS for that matter), but I think in general Time Machine also only keeps one copy of files. So APFS

I haven't done any serious analysis of Time Machine (or of APFS for that matter), but I think in general Time Machine also only keeps one copy of files. So APFS isn't worse than the existing situation. Arbitrarily keeping multiple copies blows up the disk requirements. You can intentionally keep old copies around in APFS via snapshots, which is what APFS will probably do when implementing Time Machine. What Apple added for the existing Time Machine on HFS+ was an extension of the Unix concept of hard links to files (where there is only one copy). They added hard links to directories. Generic Unix avoids doing that for a number of reasons.