Make sure while merging you are working offline in Administrator tool.
Merging originally designed to work with three repositories. As we have just two repositories we will create a dummy repository.
Go to Administration — > File — > New and create a new repository call Merge.rpd
Open the Paint Merge.rpd in offline.
Merge Paint Rpd with duplicate RPD which we created (Merge).
Go to File — > Merge and select Merge.rpd
Click Select for the Modified Repository and select Sample Sales Merge.rpd
In decision you can either select Modified or Current. As we are merging 2 different repository it is ok. select Merge. You will see an error message called : Delete Failed. Just ignore that message.
Now you will see both the repository merged as below.