Not quite. In my example of `brew install foo bar`, `FormulaAlreadyInstalledError` is raised when the current version of foo is installed. `CannotInstallFormulaError` is raised when *some other* version of foo is installed and linked:

https://github.com/mxcl/homebrew/blob/ec36776d60d3c4c92e8958df42e44d6c6e689e60/Library/Homebrew/formula_installer.rb#L78-L84