I have removed two Email Id's in the constant contact list. Then I tried to create those two E-mail id's again via API. But it is displaying the message " Email address Test@yahoo.in is already a contact ". I am following sample dotnet(C#) code which is given in the samples.
Constant Contact does not permanently delete any of the contacts you add. They will always exist as either an Active contact on lists, Removed (they have no interest lists) or DoNotMail (they requested to no longer receive any more mailings). To add a contact back to Active status on lists, you will need to do a PUT (Update) on the resource for that contact. In the .NET utility class, there is a way to update an email address. This is the method you would want to use.
Dave Berard Senior Product Manager, Constant Contact
View API documentation, code samples, get your API key.