Your asking us to identify which brand of flour you used by tasting a nibble of the bread you baked last week.
First, if the table with the client names uses an Autonumber key to uniquely identify each record, does your combo box ALSO use that key to select the matching record?
Along the same lines, the subordinate table that contains the data - does that use the LASTNAME filed, or the KEY filed?
If your using a Form/Subform to perform the return, then it creates a new record with the matching key in the subform by design. For example, if your form was an address book entry, and the sub form was phone numbers, each phone number you add you would want to be keys to the parent record.
These are just of few of the unknows here. Perhaps you should attach a sample.
Right been in this situation........ I use the same system for looking up achievement dates for qualifications.
To overcome the same surname problem i set the combo to pick up the surname and first name (initial as well) that way it can give the right information and stops problems of 26 jones ( i have 231 of them so you see ive been there) - futher more it speeds up te selection process considerably
If the table storing the names uses an Autonumber Primary Key, then the table with the additional information should link back using the key number, not the lastname. Chances are, this is how it is already setup. If not, it should be. That way, 37 Smiths will be 37 different key values.
Now, in the combo box, setup the underlying query to show:
KeyFiled, LastName, FirstName
I personally use: [LastName] & ", " & [FirstName] as one field becasue it's fancier
Set the combo box bound column to 1 and the width of the first column 0". This will make the value of the combo box the Key but hide the key from the user.
Finally, make sure the code that is finding the matching record for you is matching up the key value, not the lastname.