Alaa:
The only way to do this is to maintain the contact information in your own data collection. The CRM API is write only so you cannot read from it to validate the existence of new registrations.
What you need to do is create a data collection that contains the information that you want to monitor. Then when you are asked to register the user information you can first try to get a matching record from your data collection. If there isn’t a match then you can register the user and if successful you can add the user record to your data collection.
Here is an example of this that I put together:
Steve