Basic Access database help
I don't have Microsoft Access, but I created a test in MySQL.

My table (freddy) - don't pay attention to the name:

mysql&gt; select * from freddy;
+-------+----------------+
| regno | email          |
+-------+----------------+
|     2 | blah@blah1.com |
|     2 | blah@ah1.com   |
|     2 | blah@ah1.com   |
|     4 | blah@ah1.com   |
+-------+----------------+
4 rows in set (0.00 sec)


Then run the following query:

SELECT regno, email FROM freddy GROUP BY regno, email;

+-------+----------------+
| regno | email          |
+-------+----------------+
|     2 | blah@ah1.com   |
|     2 | blah@blah1.com |
|     4 | blah@ah1.com   |
+-------+----------------+
3 rows in set (0.00 sec)


So, as you can see, the 2 rows 2, blah@ah1.com have been deduplicated. So, you run a similar query on your customer_account_code (GROUP BY - or since you're only selecting on 1 field, you can use DISTINCT) e.g. 

mysql&gt; SELECT DISTINCT(regno) FROM freddy;
+-------+
| regno |
+-------+
|     2 |
|     4 |
+-------+
2 rows in set (0.00 sec)


Then set up your form to bring these up and allow your user to add a company_reg and company_email. 

However, this is very close to a programming (and not a DBA) question. If you have more trouble with the SQL, then come back here, but otherwise, I think you need an Access and/or Programming forum - StackOverflow perhaps?