Non-unique variable bindings in the Core

In the process of using GHC as a library, I found that the new variables bound in a core Case may not be unique, which I did not expect (what part of "unique" don't you understand?). They're in mutually exclusive branches and don't seem to adversely affect the generated code (perhaps another unique pass runs).

It's visible in the external core output, although I discovered the problem while using core data structures.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	External Core
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