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

> # Multiple columns
> vars_rename(c("a", "b", "c", "d"), c = a, d = b)
Error: Names must be unique.
x These names are duplicated:
  * "c" at locations 1 and 3.
  * "d" at locations 2 and 4.

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

