How to build an entity relationship in the Freelancer -> Customer database?

Question

Database design

Prompt, please, how to me it is correct to connect entities in a DB at a trace. conditions:
- There is a user (developer - freelancer) (a separate entity in the database - 'User')
This user, when adding a completed project, can specify the customer's data.
Actually and a question how to store essence of this customer? The user must have a key in the form of an array of customers, or should the entity of all customers be separate and independent and be bound by id to the user who added it to himself?
What's the catch - the customer can be the same for several performers, so adding the same data to each performer is stupid.
Any information and literature will be glad.

Answer 1

1) If you cannot unambiguously and automatically identify them when adding them, then there will be duplicates, reconcile.
2) Otherwise, these are classic "many-to-many" for relational DBMS:
Implemented through a separate table for relationships, something like this:

Table Customer
ID OtherData

Table Worker
ID OtherData

Table Relationship
IDCustomer IDWorker

This allows the customer to be associated with many performers.
And the performer to be associated with many customers.