Queries :: Prevent Duplicates With Unique Field Of Joined Fields

I have a client database that has recently had multiple duplicate entries. I need to reduce or negate this erroneous activity. I have a client table where I record amongst others, the following;

key
[christian_name]
[family_name]
[dob]
......

I believe that to prevent duplicate entrie via form I have created an additional field called "unique" given it as a unique index which I want to have populated with the joined fields first_name & last_name & dob (IE johndoe01/01/90), and then as user enters a new client it wont allow a duplicate.

However I need to fill all the existing customers (3600+) with the relevant joined existing data. If I create an expression I can cajoin the fields in a select query but when I try to make an update query the same syntax comes up with empty fields.

select query sql that worked to show field ...

SELECT divers.christian_name, divers.family_name, divers.dob, [christian_name] & [family_name] & [dob] AS Expr1
FROM divers;

update query that was empty ..

UPDATE divers SET divers.[unique] = [christian_name] & [family_name] & [dob];