Warn about duplicate function names

I'm submitting a ...

* [x] bug report
* [ ] feature request
* [ ] question

PostGraphile version: 4.5.4

Minimal SQL file that can be loaded into a clean database:

```sql
CREATE OR REPLACE FUNCTION graphql.replace_time_series_range(
a bigint, b bigint
) RETURNS bigint
LANGUAGE 'plpgsql'
VOLATILE
AS $BODY$
BEGIN
return a + b;
END;
$BODY$;
```

Steps to reproduce:

I am running the CLI. Here is the command line (modulo environment variables).

postgraphile --plugins '@graphile/pg-pubsub' --connection $CONNECT_STRING --schema $PG_SCHEMA --port $PORT --subscriptions --simple-subscriptions --subscription-authorization-function model.validate_subscription --no-ignore-rbac --append-plugins 'postgraphile-plugin-connection-filter,@graphile-contrib/pg-simplify-inflector'

Current behavior:

The mutation "replaceTimeSeriesRange" does not show up. If I rename the function to "set_time_series_range" it does show up as "setTimeSeriesRange".

Expected behavior:

I expect "replaceTimeSeriesRange" to show up as a mutation.