📝 Description
This PR introduces modifies the ModelPart and GeometryContainer implementation according to the conclusions reached in #12564. Long story short,

• trying to add an existing geometry (by existing we understand same Id, type and connectivity) returns the already existing one rather than throwing an error
• trying to add a geometry with an already existing Id but different type or connectivity (i.e., a different geometry with same Id) throws an error
• trying to add a geometry with different Id but already existent connectivity is perfectly valid as this is considered a different entity (current behavior)

@jginternational this should solve your current issues.
@KratosMultiphysics/technical-committee I wonder if we should update the element and condition create methods accordingly.

Closes #12564.