Duplicate type family instances are permitted

This threw me for a loop recently. To my surprise, GHC is quite happy to allow duplicate type family instances, provided that their RHSes are the same:

{-# LANGUAGE TypeFamilies #-}
module Lib where

type family Foo b

{-# LANGUAGE TypeFamilies #-}
module A where

import Lib

type instance Foo Bool = Bool

{-# LANGUAGE TypeFamilies #-}
module B where

import Lib

type instance Foo Bool = Bool

module C where

import A
import B
import Lib

f :: Bool -> Foo Bool
f x = not x

$ /opt/ghc/8.2.1/bin/ghci C.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 4] Compiling Lib              ( Lib.hs, interpreted )
[2 of 4] Compiling B                ( B.hs, interpreted )
[3 of 4] Compiling A                ( A.hs, interpreted )
[4 of 4] Compiling C                ( C.hs, interpreted )
Ok, 4 modules loaded.
λ> :i Foo
type family Foo b :: *  -- Defined at Lib.hs:4:1
type instance Foo Bool = Bool   -- Defined at A.hs:6:15
type instance Foo Bool = Bool   -- Defined at B.hs:6:15

Is this intended? My intuition screams "no", since if we offer //class// instance coherence, it seems like one ought to offer //type family// instance coherence as well. At the same time, I can't think of any threat to type soundness imposed by this (although it's quite strange to see two duplicate type family instances in the output of :i with two completely different provenances).

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture