Problem

cargo install is not idempotent on nightly (probably has never been)

Steps

cargo install xargo # OK if xargo not installed
cargo install xargo # Errors

errors with:

Updating crates.io index
  Installing xargo v0.3.13
error: binary `xargo` already exists in destination as part of `xargo v0.3.13`

but the version to install is identical with the version currently installed. This should be a warning instead of a hard error.