Hi Richard,
Thank you for the code snippet. I was able to reproduce the issue. The problem is, you are saving the file with the same name that of the source file name which is being used. Can you please change the file name in save statement? This is same as modifying the existing document.
Here is a sample for your reference:
merge.zip
Please take a look into it and let me know if you have any other queries.
Regards,
Dhivya.