For this task, you are to write a function which checks that a password is valid.

The rules for a valid password are as follows:

The password is at least $8$ characters long.

The password is no more than $20$ characters long.

The password contains at least one digit $(0--9).$

The password does not contain any duplicate characters.

Your function should print an appropriate message for each condition that is failed, and return a boolean result $($True if the password is valid, False otherwise$).$