When duplicating a post, the new one will automatically be assigned the same reference number as the original.
However, all posts in the system will need a unique reference number, which means this has to be edited on the duplicated post.
If you are unable to edit the reference number for posts:
Go to Setup, then Settings.
Click System Settings, then Global.
Search for Post Auto Reference and click this setting.
In the Boolean field, change this to False.
Click Save.
Once this is done, go into Post Setup and click on the post you need to amend. You will now be able to change its reference number.