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--ignore-existing will cause rsync to NOT copy a modified file over an earlier version of itself that is already on the backup side. This behavior would be a disaster when copying Calibre via rsync, as the metadata files can change frequently. You would want the newest one, not the oldest.
It is rare that you want this "prefer older version over newer version" behavior. The usual reason is when you want to keep a historical chain of all versions of the file. This is typical for sophisticated backup systems that use advanced rsync capabilities like linking. But it is not that common for simple rsync invocations for non-experts.
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