How to prevent duplicate entries from entering into list

How to prevent duplicate entries from entering into list

  

how to prevent Duplicated entries from entring in list given below 
i have a drop down whenever i clcik on popup widget 
it has only two entries 
company 1 , company 2 whenever i add a company 2 i should not be same be able to add same company name again ,it should give error that company name already exist ,how to achieve it 
Solution

Add a constraint to the database to make the name unique, or do a select first to check if it already exists.
Solution

i mean to say that i have only  2 records in dropdown ,and when a user click on the organization it should not add duplicated data in the table record ,like you can see in the form there has been multiple entries,,,,,,,,,,,
one user cant have duplicate organization in one table record
Hi Tarun,
You need to write a logic to the popup while adding Organisation to ckeck if that alreday added in users list ,other option is jsut dont show the oprganisation to add which are already added in the list \. to achieve this you can send a comma seprated (@selectedid) ids of selected organisations to your popup and write a advance query in the popup to get all organistains whose id not in (@selectedid).