`alr toolchain` accepts twice the same tool in the command line and eventually errs obscurely with an exception

[mosteo]

E.g.:

$ alr toolchain --select gnat_native gnat_native
error: Alire.Containers.Crate_Name_Sets.Insert: attempt to insert element already in set
error: alr encountered an unexpected error, re-run with -d for details.

We should reject it earlier with a nicer error message