From what I understand, it sounds like you will need a many-to-many relationship between the medal and user tables. In my experience, I typically handle this with lookup table that links to both the user table and medal table with foreign key constraints.
|-- UserID (Foreign Key to User Table)
|-- MedalID (Foreign Key to Medal Table)
* If medal can only be acquired once per user, make UserID and MedalID combined unique fields