Which is the SQL constraint that allows inputting NULL in a specified column in a DBMS table but that does not allow inputting the already entered value?

The SQL constraint that allows inputting NULL in a specified column in a DBMS table but does not allow inputting the already entered value is the UNIQUE constraint.

Here are the steps to use it:

1. First, you need to define the table and its columns. For example, let's create a table named "Students" with columns "ID", "Name", and "Email".

CREATE TABLE Students (
    ID int,
    Name varchar(255),
    Email varchar(255)
);

2. Next, you apply the UNIQUE constraint to the column where you want to prevent duplicate values. In this case, let's apply it to the "Email" column.

ALTER TABLE Students
ADD UNIQUE (Email);

Now, the "Email" column will not accept duplicate values, but it will accept NULL values. Each student must have a unique email, but it's not necessary for every student to have an email.