Collections: Having the same key twice in the same payload throws a lightning error

[josephjclark]

If I curl this:

 curl -X POST "http://localhost:4000/collections/stuff" -H "Authorization: Bearer $LOCAL_LIGHTNING_PAT" --json @tmp/payload.json^C

With this payload:

{ "key": "aa", "value": "xx" }

We get this exception in Lightning:

Exception:

    ** (Postgrex.Error) ERROR 21000 (cardinality_violation) ON CONFLICT DO UPDATE command cannot affect row a second time

Options:

1. Detect the issue and throw
2. Detect the issue, remove dupes and warn

I'm leaning towards 2 here