Patterns to maintain relation duplicates in relational database?

I work with Salesforce, but this doesn't matter a lot. The problem I need to solve is how to maintain duplicate records. I cannot solve this on my side, because this is comes from third party that is a Master data management. So let's say I have Accounts and Products. One account can have multiple products.

 _________         _________
| Account |------<| Product |
|_________|       |_________|

And in system I will have Peter 1 with Products "Apple" and "Carrot" and Peter 2 with Product "Orange" and Peter 3 with Product "Banana". So for Peter 1 I have to display all products mentioned here, and the same for Peter 2 and Peter 3.

I am asking for a common design practices for solutions like this, what extra objects to use to maintain relation between duplicate records. Let's say that users will fill this data manually.