Another possibility is that the RCS file was copied between the repositories at a later time.
Note though that it is possible to get this effect if a file was patched in the multiple branches, and then commited in all the branches simultaneously.
Are the original committers still around? Can they still remember what they did? In any case, it's not critical, of course, so if we presume that the original committers just were very fast and committed the same patch on different branches within one second, then by all means, simplify the graph and linearise it.