Confusing warning when fields duplicated in a schema

[joeapearson]

This isn't really a bug per-se but a usability issue, and was entirely caused by my own mistake:

I suddenly found after a series of changes to my code that I was getting warnings like this:

warning: this clause cannot match because a previous clause at line 1 always matches
  lib/myapp_web/schema/my_types.ex:1

It turns out I had mistakenly added duplicate fields to one of my schema. The only way I could track it down was by progressively commenting out bits of my schema until I isolated the warning.

Environment

• Elixir version (elixir -v):
  Erlang/OTP 23 [erts-11.1.8] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [hipe]

    Elixir 1.11.3 (compiled with Erlang/OTP 23)
  

• Absinthe version (mix deps | grep absinthe):
  * absinthe 1.6.0 (Hex package) (mix)
  locked at 1.6.0 (absinthe) 99915841
  * absinthe_plug 1.5.4 (Hex package) (mix)
  locked at 1.5.4 (absinthe_plug) 80360cd8

• Client Framework and version (Relay, Apollo, etc): N/A

Expected behavior

A warning or error of some sort that informed me that I had made a silly mistake!

Actual behavior

I receive several warnings like this:

warning: this clause cannot match because a previous clause at line 1 always matches
  lib/myapp_web/schema/my_types.ex:1

The line number is always 1.

Relevant Schema/Middleware Code

Here's a minimal example that reproduces the behaviour:

object :my_object do
    field :my_field, :string
    field :my_field, :number
  end

Of course this is a contrived example because you can clearly see the mistake when there are but two properties. If your object schema is larger (like mine is) then it's harder to see. It'd be great if some sort of warning and or error was triggered (with the appropriate line number, instead of just line 1).