Deduplication is at block level, so local changes anywhere (including file end) only save updated blocks, which would save space, however as blocks go out of use (old backups age out), the unused space isn’t immediately reclaimed, but waits for an automatic-by-default version of The COMPACT command to run.
If you are saying you have two static files that only differ at the end, the backup of the second file would realize that the initial blocks already exist, courtesy of the first backup, and reference the blocks instead.
If you want more technical detail, click on the ARTICLES link on the fact sheet. Good ones might include: