Duplicate, unused variables generate syntax errors only for the second occurrence.

[Idorobots]

Running the following code:

(let ((x 23) (x 5)) 23)

...produces the following errors:

expr(1,13): Bad `let` bindings syntax, duplicate binding identifier `x`:
  1 |(let ((x 23) (x 5)) 23)
    |             ^^^^^
  2 |

expr(1,14): Unused variable `x`, rename to `_` to avoid this error:
  1 |(let ((x 23) (x 5)) 23)
    |              ^
  2 |

expr(1,14): Unused variable `x`, rename to `_` to avoid this error:
  1 |(let ((x 23) (x 5)) 23)
    |              ^
  2 |

The first "unused variable" error should point to the first occurrence. If not possible, both errors should point to the first occurrence (and be deduplicated).