How i can prevent from repeating 'unit_name' in table

Are you trying to insert values new values from datagrid?

In that case you are not updating your datatable with new values in the above code.

Your code is set for inserting values from your datatable. here as many clicks user do as many times values will be inserted from datatable.

Here your code is set for something and your intention is different. you have to refactor the exiting code as per your requirement.