This is not possible as of now with GNU tar, but things exist :
Note that hardlinking and deduping does not have the same semantics, one would need another kind of tar node type to represent "dupe data" in order for the archive extraction process to properly recreate duplicate (and thus indendently-living) files; that would create incompatible tar archives with most standard tools (GNU tar, pax, etc) which would be a bold move.
Let me insist : if you were able to assimilate dupes with hardlinked files, you would have a problem at archive extraction :
* are they really hardlinked files like those of a Git repo ? Then they should be really recreated as hardlinked files otherwise the restored Git repo won't work ?
* or are they really identical files, and if they were restored as hardlinks, the restored archive would probably have major data leaks (imagine you have assimilated identical /etc/passwd files while archiving, restore them as a single file with multiple hardlinks : one modification in one VM becomes visible in the other!)