I a in the same situation.
I got two lists, and if a contact is new it does add the details to the one list. But if the contact is in the other list, it says that the email address is already there.
This is some process that t I found in the forum: (Tom Mignosa, Product Manager, Constant Contact)
Use the Contacts Resource to find out if the contact already exists in the database (see "Querying the Contacts Collection" section of that page).
If the contact already exists, the "Query by Contact" will return a link to the Contact Details (ie. a URI). GET the contact details for the contact, which will include the list association for that contact. Update the returned XML as needed and PUT the contact XML back.
If the contact does not exist, the Query by Contact will return an empty set (a XML stream with no tags). Construct the entry to create the contact, including list associations, and POST the XML back.
unfortunately, I don't know how to integrate that within my existing php code.
The samples (in php), on the page that you gave above, are not helpful for that particular matter.
Could someone help me out ? some php samples?
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