Why is XCode giving me the error 'Reimplementation of class'?

Question

XCode gave me a very unhelpful error about not being allowed to re-implement a class. I searched and I searched, and nowhere else was that class name implemented -- referenced, but not implemented.

I actually did a keyword search and went through every single line of my project that referenced that class, and didn't find anything that tried to implement it. (And, annoyingly, there's nothing online that helps with this particular error -- hence why I'm including the self-answer, to help the next person this happens to).

Answer 1

The key, as I eventually discovered, was in a malformed include. I hit the wrong button when using autocomplete, and another class was including the .m file instead of the .h file -- with the result that I was, in fact, implementing the same class twice. Not because I was doing it in two places, but because I wound up actually trying to compile the same file twice!