General :: To Know If Same Value In The Field Already Exists

I have a table called 'maintable'. It has 5000 plus entries of employees with different fields. Every Employee has unique ID. One field is date of birth (dob). Normally more than one person can have the same date of birth. A form (maintable1) is created to enter data into this table.

In order to avoid duplicate entry of the same employee again with all his data, I wish to know if the same date of birth (dob) which is being entered now already exists for any employee to verify if the employee being entered is the same, if not then I can enter the data of new employee with the same date of birth.

So basically when I will enter data in a form for the dob field, if the value is the same, then a message that entry already exists, then the 'maintable' may open highlighting the same date of birth entry, so it can be verified if the same person is not entered again.