Please understand my reply here is “mild,” I’m not “pounding my shoe on the podium” about this.
There really are synonyms in both natural language and the resulting tags that end up in OSM. The reasons for this range from the obvious: somebody decided to tag “graveyard” instead of “cemetery” for example, and didn’t read our wiki on whether TagA or TagB would be more appropriate for a given semantic, to the obscure (regionalisms in English dialect, very slight differences in how something works mechanically, the meaning of a sign giving rise to different key values…).
But if we “blithely” mechanically conflate TagA and TagB (and there are many examples, even some including TagC and TagD), we would lose such subtly. “But,” you say, “how will a renderer grab a hold of a (single) tag and properly render?! Oh, my!” The answer is, conflation of multiple tags “down to” a single tag isn’t the job of OSM (Contributors) to “make easy by being a single tag,” it is the job of the renderer to say “well, graveyards and cemeteries are going to be rendered identically in my renderer, so I must gather both and assign them the same icon or border-fill, or whatever graphic / semiotic they’re going to get.”
If there is another reason you wish to “consolidate” or “clean up” such “synonyms,” I’m curious to hear what it is. I’m in listening mode, not “mild admonishment that might not be a good idea to do that” mode.
Thank you for seeking the “lay of the land.” That’s all I’m offering with this post, a simple sketch. And your initial reasons are good, though I do wonder if how things render affects why you are asking.
Edit: Our “whatever tag you wish” tenet of “plastic tagging” notwithstanding, I do not mean to convey by this post that OSM Contributors can lazily tag however they want to. Of course, especially if you are new to OSM mapping and there IS a tag for what you are mapping, it is correct (even preferred) to use that tag. This often means consulting our wiki for proper practice, a designed-to-be-easy-and-painless method of documenting our current tagging practices (and sometimes, future strategies to better tag). As time goes by, newer, fresh mapping/tagging (especially by novice mappers who might have a tendency to introduce “non-standard” keys/tags) better aligns with existing tagging, while simultaneously, “best tagging practices” evolve. Eventually (someday), it will be clear and obvious what to tag nearly all of the time, while we continue to allow plastic, novel tagging. Today, we’re only part-way there, but it does get better.