Active Duplicate

I have hypothetical question on a scenario and hope some of you could share your thought.  If I use 'active duplicate' to create a standby database while the primary is up, which is what I am counting on, how does it work if data gets updated in primary before the active duplication is done?  In this case, what do I expect to see if I continue to complete the standby database setup process on the new standby by adding it to the dataguard configuration?

Thank you.

Martin