How to create medical bill generation

hi all, I am trying to generate medical bill like below image    I used input reference selector to select the product all working fine but, if i enter values for amount ,quantity and all .its saving in database    for example : Once i save and  if i select okacelt-l [productname] second time i need edit the amount ,quantity values  so this is the issue  so how i can solve this issues ?     or  is there any other way to generate bill?   thanks
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