Dear Vijay,
Thank you for your answer. However I don't think the issue is the actual key of the item, which is added to queue.
For key I use completely different string, different column of the input collection. And I can have multiple items added to the queue with the same key without any problem.
The value, which is problematic for blueprism is the datetime value, which defines when the item was added to queue. And this is created automatically by Blueprism