Allow duplicates from web API, merge later using "Deduplicate Leads"?

Allow duplicates from web API, merge later using "Deduplicate Leads"?

I have a Zoho CRM database containing a custom "License" field. Prospects go to our site and submit a form with more, updated information and also the License field. 

My problem: 

1) If I make that field Unique in Zoho CRM, the API will not pass the record as that License already exists.
2) If I allow duplicate records, I have no way to identify them as Zoho will only search Email, Company, Phone or Mobile for duplicates.

Question: "How can I allow duplicate records to be created (each record contains unique information) and later find and merge the duplicates?

*Ideally, I would like to match/update based on that License field (i.e. if it exists, update. If not, create new Lead) and understand this is not possible.