Two Contacts can be merged together using the Merge function.
The merge function works by making all data on the "primary" record take precedence over data on the "secondary" record. Where data doesn't exist on the primary record but does exist on the secondary record, the data will be added to the primary record when the two are merged.
How to merge Contacts in the OLD profile page view
From the Contact profile page, click the Merge button under the spanner icon in the profile page.
How to merge Contacts in the NEW profile page view
From the Contact profile page, click the three dots next to the green Add dropdown menu and select the Merge function.
The profile page from which the spanner icon was selected will auto-populate as the primary Contact. Use the secondary lookup field to locate the Contact which should be merged into the primary. The Contacts can be swapped to change which profile page is the primary and which is the secondary.
Click the Merge option to complete the action.
There will now be one complete Contact record within the system.