It is impossible for me to give the level of guidance you are asking for with the level of detail you are providing.
You are providing a high level overview of your database structure, with this information, I can only provide a high level overview of the steps you need to preform.
If there is a specific step you are falling down on, I can help you with that, when you ask instructions on how to accomplish a task, I can really only give guidance on how to accomplish it, not the step by step instructions you are seeking.
from the error you provided:
"Duplicate entry '' for key 1"
it sounds like the product_clientid column in your table was created to not allow duplicates. If I am reading correctly, this column is in the Products table and is the foreign key relation to the clients.clientID column. make sure the that product_clientid column can accept duplicate values.
The key information you need here is this:
1) The clientID is stored in a session when they login.
2) To return records for that client, create your recordsets to filter the product_clientid column on that session variable.
3) To create records that relate to that client, store that session variable in the product_clientid column.
If you need specifics on how to create a recordset that filters on the client ID session variable, look at the recordset created by security Assist on the profile page.
If you need any other specifics, please feel free to ask