Giving, Groups, and forms use the first and last name combined with the email address to determine if a profile matches one in your database. All three fields must match exactly, or the system will create a new profile. It's safer to create a new profile that can be merged with the correct person than to add the wrong person's information to an existing profile. However, that can still create duplicates for you to merge.
In Registrations, if someone registers a person not in their household, a duplicate profile will be created even if the registration contact uses the same name and email address as their existing profile.
Here are some ideas to help people stop creating duplicates:
Encourage People to Log In
When a person is registering for an event, making a donation, signing up for a group, or filling out a form, they are doing this from the Church Center website. In order to ensure their information is attached to their profile, ask them to log in.