Silently ignore input when alternative is already active

[TAAPArthur]

Previously kiss a would error if the alternative provided was already in use. Now such a case is silently ignored. This is useful when there is a post-install kiss_hook automatically and intelligently swapping files using preferences pre-generated with kiss-preferred.

[dylanaraps]

I think it's best that users manually manage their alternatives to understand each modification that is made and the implications of doing so. The feature was not designed around being run inside scripts.

Thanks for the PR.