I have many2many field combined with tree view:

<group>
    <field name="cpu_ids" widget="many2many">
        <tree>
            <field name="name"/>
            <field name="frequency_base"/>
            <field name="frequency_tb"/>
            <field name="cores"/>
            <field name="threads"/>
        </tree>
    </field>
</group>

And when I choose one particular record I can not pick it again for the second time. 
In this example I would like to be able to pick the same CPU couple times.

How do I do this?