You need to create a separate entity for this. Create a separate entity and add 1 attribute with the name you want to remove the duplicate values. and associate this entity with your main entity. after this create a new microflow and retrieve data from your main entity and then add a loop for your retrieve activitiy and in loop first create retrieve activity for your new entity use xpath as your duplicate value attribute. Then check if retriever is empty then create new object of your new entity and commit although use change object activity for iteration and set your association as your new object  and in false condition means that this data already exist then you only need to use change object activity for iteration and set your association as your retrieve object. after all these you need to add reference selector in your page instead of dropdown and select attribute of your reference select as your new entity attribute