After updating the value in Database, you are just clearing the textbox and not deleting the textbox. So that's why it is coming as 6 textbox's (existing 4 and new 2) when you click the button again. So it's better you delete the textbox after updating the values in database. So that it will not be 6 when you click the button again.
For deleting the textbox please refer this link
Please let me know is this what you are searching for !!!
Happy Coding :)