How to create entities that share

I need a many to many relationship between entities, so i created a third entitie in the middle like a should.
The thing is, in the training videos it is shown that an index should be used to prevent having multiple instances of an entity with the same combination of  foreign keys. In my case however, I want to have the possibility to duplicate. I've tried not using the unique index, as well as using it but setting it to "not unique".  The platform however doesn't allow me to create intances the way I want...
I've created some many to many relationships and can't find a problem.
Is your problem in the Entities or in the screen? Making a screen diferent from the standard can be tricky.


Can you show us why you can't create the instances? Is there an error message? Yes, when, while publishing?
Thank you for the repplies.

This is the error message that I'm getting

Those are validation messages not put there by the platform. Check the action for the 'Guardar' button.
Thank you, it is solved