Domain duplication should take multiple inames as arguments

Suppose there's a non-box domain (say in `i` and `j`) that is intended to be "cloneable" via iname duplication. Cloning `i` then `j` does not attain the right result.

Start by reproducing the reported case: a non-box domain involving inames i and j, with duplication applied first to i and then to j. Trace the domain-duplication entry point and compare that result with duplication that accepts both inames together. Done means multiple-iname duplication produces the intended cloneable domain.

Written by the indexing model from the issue text.