In the case of a conflict when sending unmodified data, the most likely cause is a contact in a Confirmed state. This is because the PUT action always updates a field, even if the input is the same as the previous value, and the email address of a Confirmed contact can only be changed with an action by that contact. If you're working on an implementation that handles groups of contacts, I would definitely advise making use of our bulk APIs, as they do not have this same limitation as a result of being unable to change email addresses.
You can find information about our bulk APIs here: http://developer.constantcontact.com/docs/bulk_activities_api/bulk-activities-import-contacts.html
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