We've tested this in Firefox and Chrome. It only seems to be an issue with Microsoft Edge.
Microsoft Edge appears to create a .url file when you drag a link (such as an activity title) and then creates a resource for that .url file if you drop it into the course. This is a bit confusing as, at a glance, it looks like you're duplicating the activity.
Attached is a video of the issue.
Go into a course as an administrator.
Turn editing on.
Drag an activity title.
Drop it below the activity so that it uploads.