Anyone got a clue about this one - and how to reverse rather than redo the whole thing?

From a saved mail-merged document of 50 letters, I saved as 50 single 3-page documents and then moved in smaller batches to 3 different folders.
Opening the first folder, to work with the documents, made sure (knowing Word of old) I checked each to make sure they contained different contact information as per the saved document name. They did.

Opened the first folder and first file, selected File/Save and Send/Send as an Attachment to send as an email attachment. Proceeded to do the same with all files in the folder. All went as planned.

Opened the second folder and first file and did the same. Was fine. However...opened the second file and the content now matched that of the first document.

On checking, every single document (which had been closed and untouched since saved and checked the previous day) in that folder now matched the first one.

Hours of work to be redone...