Check for duplicate index keys after converting legacy index definitions

If a dump has two indexes, {foo: ""} and {foo: 1}, and we restore with --convertLegacyIndexes, then we will get an error: 

(IndexOptionsConflict) Index with name: foo_1 already exists with a different name

We could check to see if we have duplicate indexes after legacy index conversion and discard the legacy index, or we could just ignore the error.

Both options would have the effect of removing the legacy index. This would mean that index hints for that index would no longer work in the restored db.