Backing Up multiple disks/computers to one Time Machine Disk.

<< I hope this also applies to duplicate files on multiple disks? >>

Sorry, NO.

• There is no cross-checking between backups of different computers.

• Also, if you have copies of a file on Different drives of the same computer, multiple copies will be saved, one for each occurrence on a drive.

For MacOS regular drives, Apple File System (APFS) will not literally duplicate a file on the SAME Active drive. it will instead create an additional File System pointer to the original copy. But this is NOT done on Backups, because the files are not seen as duplicates of the SAME file.

The punchline: For compact backups, be sure to prune the ORIGINAL sources, because Time Machine does NOT make multiple instances of files more compact automatically.

Backups for a computer are stored in a sub-directory on the backup drive, identified with the computer-name.

Subsequent backups of the same computer may add additional unique files that are not already on the Backup set for that computer.

if you exceed the allocated space, backups may be consolidated (NOT deleted). If you greatly exceed the allocated space, the oldest backups (but not the files still in use on the current Mac) may be eliminated -- these are NOT Archives, they are Backups.