This is a workaround for Office/Outlook/Word 2013. Office keeps an index of the last 100 files that Outlook has sent as attachments. It is used to coordinate merging. Find it at:
File is named "AdHoc.rcd"
Prior to making changes, I suggest making a backup of this file in the same directory; for example "AdHoc.rcd.old" leaves a clue behind that this was the original.
Open the file with notepad and delete all of the text. Save the file.
NOTE: ".rcd" isn't recognized as a text file by Windows, so you'll probably have to tell the system which program to use to open. If you cannot see the file extension at all, you will have to set Windows explorer to show those. There are many resources on the internet for doing so.
Right click "AdHoc.rcd" file and select "Properties"
Under the "Attributes" section, place checkmark in "Read Only"
This prevents Office from recording file names/locations for tracking, which eliminates the message offering to merge. This disables a collaborative feature of Office - use this workaround at your own risk.
Many blog posts and a few articles on the net refer to a page which no longer exists:
If anyone has a cached version of this page it would be helpful to post this link in the comments.
A good explanation of the background and use of the AdHoc.rcd file is at:
Many posts I've read pertain to Outlook 2002, but the file is still there and seems to function in the same way in Office 2013.