Strange behavior when pattern variables are duplicated on pattern synonym RHS

These might be two separate bugs, but they're similar enough that I'll bundle them under the same ticket.

If you take a pattern variable bound on the LHS of a unidirectional pattern synonym and use it multiple locations on the RHS, then GHC accepts it, surprisingly:

pattern Foo a <- (a,a)

What does Foo do? As it turns out, the right-most occurrence of a in the RHS seems to take precedence:

λ> :i Foo
pattern Foo :: b -> (a, b)      -- Defined at <interactive>:12:1
λ> case ('a', 'b') of Foo x -> x
'b'

I expect the definition of Foo to error.

When duplicate pattern variables are used in the RHS of an implicitly bidirectional pattern synonym, then it does error. However, the error message is quite misleading:

λ> pattern Foo a = (a,a)

<interactive>:16:17: error:
    Invalid right-hand side of bidirectional pattern synonym ‘Foo’:
      ‘a’ is not bound by the LHS of the pattern synonym
    RHS pattern: (a, a)

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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