[Haskell-cafe] conflicting variable definitions in pattern

It is pretty clear, that the following is not a valid Haskell pattern: foo (x:x:xs) = x:xs My questions is _why_ this is not allowed. IMHO, the semantics should be clear: The pattern is expected to succeed, iff 'x' is each time bound to the same term. Isn't this allowed, because this would require a strict evaluation of the 'x' variables? Thanks, Martin