This compares disk usage of a backup I made of /home two years ago with one made this month. There's a lot of overlap - more than I expected.
Method: creating the second backup, I used rsync with
--link-destspecified; for measuring, I used du -hsc old new; du -hsc new old. The size of the overlap is the size of new given by the second du minus the size of new given by the first du.
I'm trying to figure out ways of showing overlapping disk usage for more than two or three rsync trees created in this way (for example, to visualise monthly, weekly and daily backups over a long time period)