So, when my data model is ready, I tried to generate the subclasses when my data model file is open:
Editor -> Create NSManaged Object Subclasses
Okay, two files are generated, one is class declaration, the other is the property.
However, I was immediately prompted that I did a redeclaration of a class that already exists. This is where the original generated files are:
previously generated files
My question is obvious:
You are generating files which have already been generated for you by Xcode and thus get duplicate declarations.
Find detailes in this answer.