I don't think there is a way to do this within Marketo forms. You might need to use one of the APIs to pass the leads in uniquely. If you are using SFDC, you could create a web to lead form and pass it into SFDC which would then sync into Marketo. Also, you could explore custom objects for this type of situation.
Keep in mind that even if you are able to get them logged as two or three leads there will be limitations to what how you communicate with them (ex. an email program sent to the three dupes will only hit one of the leads submitted infromation).