Textbox value Problem

Hello All,

I have two Textboxes and one button(called : ADD)

I am entering value 1 in Textbox1 and press button ADD, so now that value of Textbox1 Enter in Textbox2 with comma(i.e. 1,)

now again i am entering Value 1 in Textbox1 and press Button ADD, So now in Textbox2's value will be 1,1 and so on...

ex : 1,1,2,3,3,4,4 --> i need is : 1,2,3,4

 now my problem is that i dont want to repeat value in textbox2, i.e.1,1, so when user add same value it gives error.

 

Please help me out.

 
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