When I try to activate an environment that is already active I get an error:
± flox activate -e rust
ERROR: /Users/etorreborre/.local/share/flox/environments/etorreborre/rust environment already active
Since the operation is idempotent and not breaking anything maybe a nicer informative message would be enough:
/Users/etorreborre/.local/share/flox/environments/etorreborre/rust environment already active