John M. Dlugosz writes:

I'm getting a strange template instantiation error with a simple and reasonable use of multi_index_container, which I've written up in detail with a minimal program to reproduce here: http://stackoverflow.com/questions/34450642/ why-is-this-member-function-a-duplicate

I've answered there, the problem has to do with misplaced const qualifiers in the specification of a composite_key.

This showed up in Visual Studio 2015, and has worked just fine for years on VS10 and on other platforms. I've tried updating Boost and new versions don't make any difference [...]

I don't know why this worked in VS10, it certainly should not and definitely does not work in GCC, which I tried before posting my anwser. Joaquín M López Muñoz Telefónica