Manage N:N relationships

When managing N:N relationships in TAB relation, how can I exclude items that are already defined in the relation tp avoid insert thems twice?
thank you for your help.

In 1: N relations this control, side 1, is already present.
In the N: M relations the control is not implemented precisely due to the fact that in both sides there are no limits to the number of cards that can be connected, and a card can be connected to several other cards with that relation.

Thank you for your help