"Your password is too weak"
Yes, this would suffice. But I think it points out that a duplicate password is just a proxy for a weak password: strong passwords tend to be locally unique.
If you are going to have a strength requirement, then run your strength validation routine and deny weak passwords. That a password is duplicated seems like a special case of weakness that is not worth checking for.
i.e. You probably don't want special checks for 'password' or '123456' either, since your strength validation routine should catch these.
"I'm sorry, another user on this system is already using the password you have submitted. Please choose a different one."
That's not the sort of information you want to be leaking.