It would be great if we could specify the unique field to use as the key for duplicate checks.

We have customers who want to use their own fields as keys for modules such as Accounts and Contacts. Using the API to sync with other systems would be far easier if we could use these fields as keys, instead of being forced to use and email address or Account Name.

I know this is possible using the SDKs, so I can't imagine adding this functionality to the URL APIs would be too much extra work.