Lots of discussion, a plugin, and even patches in MDL-24443. Also very highly voted-on.

The MUA also put this forward as a proposal in MDL-61542, and there is thorough documentation on that ticket.

In addition to building out the utility to actually merge two accounts, we should build out a "suggested duplicates" screen, to allow administrators to proactively deal with this, and also to give them confidence in their merges.