Start by creating a vector to hold a unique values of the input range.  We
expect that in most cases, there will be relatively few duplicates, so
reserving enough memory for a complete copy is worthwhile.  Unlike a solution
involving a std::set, this algorithm allocates all the memory it needs
in one go and in one place, so it is more cache friendly.

Populate the unique copy with a lower_bound insert algorithm and record the
indices of duplicates.  This is the same complexity as the std::set insert
algorithm, but without the need to allocate memory on the heap and other
disadvantages of std::set.

Remove the duplicates with the cmRemoveIndices algorithm.