This issue only appears if the let declaration is inside a block. If it is a direct child of a function body/program, babel throws correctly.

REPL

Input Code

{
  let a;
  { var a; }
}

Expected Behavior

Duplicate declaration "a"

Current Behavior

No error

Possible Solution

Scope should check the block scopes that variable declarations visit while they "walk up" the scope.
It should also keep track of them, in the case that the variable declaration comes before the lexical declaration (e.g. { { var a; }; let a; })

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