restricting user to enter only one Primary care physician during enrollment

Summary:

We have added a new HMO plan this year which let's user enter PCP details during enrollment through action item, current functionality lets the user enter multiple PCP details either with same specialty or different. we want to restrict user to enter only one PCP info, I found the lookup table to take out extra specialty values from drop-down but I need help to restrict user from entering multiple PCP info with the same specialty.

Any suggestions/help is greatly appreciated.

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