Doubled values with Lookup field

In table 1, I will collect the personal data (+ product data) of customers who want to advertise the purchased product via the form.
In table 2, I would like to keep a list of all customers (with all personal data) + which complaint cases they have + the total sum of all complaints they have (per customer).
How to connect the tables to each other so that all personal data that comes from the form (in table 1) is transferred to the customer table?
I already tried with lookup fields, but if the same customer has two different complaint cases, the values are doubled.

https://airtable.com/appAHXw6BaUIsoBE5/shrjZYXs9PpqyquLy