Duplicate bindings not discovered at parse-time

[pdarragh]

In Racket, duplicate bindings in a let are a syntax error:

> (let ([x 1] [x 2]) x)
string:1:13: let: duplicate identifier
  at: x
  in: (let ((x 1) (x 2)) x)
 [,bt for context]

We don't perform these checks during parsing, though, and instead wait until interpretation or compilation to discover the issue. Historically this has been fine because we simply don't test syntactically invalid inputs.

However, as we continue to implement competitive elimination assignments with reference machines, it becomes important to improve the underlying implementations. I think it would be best if we incorporate checks for this in the parser going forward.