You sent a document to someone as an attachment. Both of you made changes. Now you have two copies of the document. Learn how to compare two versions of a document in Microsoft Word – without copy paste or manual scrolling. Amazing feature in Microsoft Word. Compare Documents option – Review tab.
Microsoft Word Track Changes Masterclass – in-depth
How to Protect Track Changes in Microsoft Word
Who changed my file? – Track changes – OneDrive Link Sharing – Word / Excel / PowerPoint