Impact of unique partition key on DynamoDB performance
Solution 1:
In your case it is okay to duplicate the partition key as you are using a sort key too, if you only use the partition key with duplicate values you will have issues in dynamoDB partitions the data.
DynamoDB stores and retrieves each item based on the primary key value, which must be unique. Items are distributed across 10-GB storage units.
For tables with composite primary keys, the sort key may be used as a partition boundary. DynamoDB splits partitions by sort key if the collection size grows bigger than 10 GB.