> # Renaming to same:
> vars_select(letters, foo = a, bar = b, foo = c, ok = d, bar = e)
Error: Must use unique names when renaming columns.
* Columns `a` and `c` are being renamed to `foo`.
* Columns `b` and `e` are being renamed to `bar`.

> # Renaming to existing:
> vars_select(letters, a = b, ok = c, d = e, everything())
Error: Must use unique name when renaming columns.
* Column `b` is being renamed to existing column `a`.
* Column `e` is being renamed to existing column `d`.

