how can I restrict the users not to enter the balance again for the same item code?

I wanna make a userform for entering each start balance for each item code. (total 60 item code, each item code only has one start balance)
For examples, the start balance of item code A is 700, so when I choose the item code A and enter 700 for the start balance, then the record is done.
Thus, how can I restrict the users not to enter again for the same item code?

Thanks,hei

Here is my code, but not correct

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How are you storing the data?
Could you upload an example sheet?

U can see that in sheet 2, the start balance of product A is 5000.
All product only has one start balance,
so I dun wanna the user can use the userform to enter the start balance for the same product again.

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textbox_startbalance is the textbox of start balance, and I put this code in the userform.

I have changed the code a little bit, but still cannot restrict the users enter the balance again for the same item code

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