scenario

Hi,

I have one Businessproceeflow it has three steps on that third step when i enter a date field one appoinment willbe created if i do it workflow when i enter same date again and again same appointment will be created because onchange we have doing but i want to avoid it when i enter a second time i need to show one message already appoinment will be created.

how to achive this using plugin ? let me  know anybody .

Thanks,