How to implement properly a view of nodes displaying other referenced nodes with no duplications?
You can fetch the 'Parent' nodes with their 'Children' nodes with the default functionality of views and without the use of Relationships, Aggregation or other modules.

The Steps Are:


Create a view of 'Parent' content type.
In the 'Fields' section add the field 'Content: Title' and the field 'Content: Children(Appears in: node:parent)'.
In the 'settings' of the 'Format' section  select as 'Grouping field Nr.1' the 'Content: Title' field.
In the field 'Content: Children' select the display format that you want. 
Finally 'Exclude' from display the 'Title' field.


Enjoy