On the 'Contact' search screen, perform a search that will display both records in the same list.  Don't open either record, but tick both contacts on the left hand side. Right click on the record you want to keep and select 'merge as master contact'.  (if this option is missing or greyed out, speak to your superuser).  The subsequent screen will show you which data will be retained and which will be lost as part of the merge.

If both records have the same type of data, e.g. phone number, the data from the master contact will be retained and the other data will be lost.

If the secondary record has some data that is missing from the master record, e.g. date of birth, that will be retained in the new record.

Any links to other records, e.g. tax returns, will be merge as well so no links will be broken.  This is a very good reason to merge duplicate records rather than delete them.

Note:

• You can merge two contacts (and only two at a time)
• You can merge a client and a contact (if the contact is kept as the master record any tax returns or accounts will be lost)

You cannot merge two clients