How to populate options of a drop-down field with entity data?

[Bruno Busco]

Hi,
I would like to write a form widget where the user can select all products
that fulfill some constraints.
The constraint should be input using some drop-down fields.
With the following code I get that in the drop-down field the same option is
displayed many times (because there are many records with that value) while
it should be only displayed once.

        <field name="diamId" title="Diametro" position="3">
            <drop-down allow-empty="true">
                <entity-options description="${diamId}"
entity-name="Product">
                    <entity-order-by field-name="diamId"/>
                </entity-options>
            </drop-down>
        </field>

Can someone tell me how to populate the drop-down options having each option
displayed only once?

Many thanks,

-Bruno