How to clone model and it's relationship

I have news model that has categories with many to many relation.
when i clone this model, it attributes copied, but relations are not copy!
this is my code:

 $duplicateNews = clone $this;

 $duplicateNews->id = null;

 $duplicateNews->isNewRecord = true;

$this->categories exist and has value
but $duplicateNews is not exist
what should I do?
please help me.

I don’t think it supports deep cloning…