How to prevent users from creating repeatative ids?
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In my web application I have a simple employee form. Each employee has a unique id associated with it. 

When a new employee is added to the system, a unique id is assigned to him. This id can be assigned manually or can be generated by the system. 

We have a table where we have a single column with a number stored in it. It gets incremented by 1.

So if we have the table has a value of 10. System will auto generate an id of E0011 for next employee.   

And the value in the table is updated to 11. 

We have a button near "id" textfield. If user clicks on this button a new system generated id is created and populated in the id field.

Now, the problem is user can click on this field multiple times and generate the next id so the previous id is lost or consumed. 

How do I prevent user from createing multiple ids?

