compiler/add_mutable_aux_preds.m:
If a mutable has the same name as another mutable defined earlier,
report this fact, and don't try to implement the mutable. If we did
try to implement it, we would get a whole slew of errors about
multiple definitions of all the other kinds of items that we turn
a mutable definition into.
tests/invalid/duplicate_mutable.{m,err_exp}:
Add this test case for the diagnostics we generate for this issue.
tests/invalid/Mmakefile:
Enable the new test case.
