How should manual package `install` handle package already being installed

[peter-bread]

Problem

If a package is already installed, how to handle calling the install script again.

Related: #93.

GHCup

GHCup provides an option to handle whether or not to upgrade existing installation.

https://github.com/haskell/ghcup-hs/blob/4c6cbb0c47c072cc9deb3c55509e1ef2e272102d/scripts/bootstrap/bootstrap-haskell#L979-L985

Rustup

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