How to reproduce : 

We are working with a zip file file.zip containing 2 files fileA and fileB. 

1 - In module file, drag and drop file.zip and choose to unzip file. 

=> fileA and fileB are added. 

2 - Drag and drop file.zip and choose to unzip file. 

=> fileA and fileB are updated. 

3 - Received notification mail is titled "Documents added", but is should be "Documents updated"