Duplicate shard detection cannot be turned off

The duplicate shard detection logic is always on and cannot be disabled. This was a part of the original design for the schemarouter where the shards were strictly limited to being on only one server. With advancements in the schemarouter, it no longer makes as much sense to be this strict which is why the option ignore_tables_regex exists: it can be used ignore some tables that are allowed to exist on multiple servers. However, it's also the only way to turn off the whole duplicate shard detection which is why it's not too uncommon to see ignore_tables_regex=.* in a configuration that uses the schemarouter.

The option should be made configurable with a simple boolean option like detect_duplicates. It could also be made opt-in instead of opt-out but this would be a change that silently changes the configuration so it shouldn't be done without consideration.