Unavoidable duplicate constraint warning

The "duplicate constraint" warning can be triggered in a context where it's impossible to remove it, because the constraints come with a type synonym.

The following standalone little module reproduces the problem:

{-# LANGUAGE FlexibleContexts, Rank2Types #-}

module Test where

type Constrained x y r = (Eq x, Eq y) => x -> y -> r
 
f :: Constrained String String ()
f = undefined

The warning is

Test.hs:7:6: Warning:
    Duplicate constraint(s): Eq String
    In the type signature for `f': f :: Constrained String String ()

The warning is present in 7.4.1 and 7.6.1, and presumably in the versions between as well.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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