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Do you want to disallow the signing itself when user tries to use same credential to sign the second signature field or would you want to reject the PDF form in your workflow when performing validation on both signature fields?

If it is the latter you can use LiveCycle Signatures validation APIs to ascertain if same credential has been used to apply digital signature on both the fields.

If it is former you can add a script in presign event of second signature field which validates the credential info from signature field 1 and if both are same then pop-up a message and cancel sign operation