> # New column
> vars_rename(c("a", "b", "c"), foo = a, foo = b)
Error: Names must be unique.
x These names are duplicated:
  * "foo" at locations 1 and 2.

> # Existing column
> vars_rename(c("a", "b", "c"), c = a, c = b)
Error: Names must be unique.
x These names are duplicated:
  * "c" at locations 1, 2, and 3.

