Merging two contacts
To merge 2 contact records, select both contacts from a list then select the action 'Merge contacts' from the 'Actions' drop down.  In the resulting screen you will see there are several options that you can check/uncheck - one of the most useful ones is the option near the top of the screen to 'flip' the 2 contacts - the left hand one is regarded as the duplicate.  Also, the deleted record goes in the trash so if you make a mistake you can always retrieve it.