Surely only history is needed. The "of Islam" can be assumed. I think the two smaller tags should be edited to history by hand, but history-of-islam should probaly be delt with by moderator tools to avoid flooding the front page with a bunch of revised posts.
I've chosen to just merge the whole array since I can't think of any conceivable (on-topic) need to keep the tags separated. Didn't bother to make any synonyms, since one would need to go through "history" to get to "history-of-islam" anyway, whereas the "islam--history" and "islamic-history" obviously don't get enough play to worry about.