In Windows 10, what is the difference between <user>\Documents and This PC\Documents?

Those two are the same folder, and it only appears once. The paths that Explorer shows on the top can be a little misleading- they're meant to make it easy to find files, so a lot of important stuff is omitted. It also tends to be inconsistent, so your Documents folder might show up under My PC or by itself at different times. To get the actual path to the file/folder, right-click on it, select "Properties", and look in the "Location" section. In reality, your Documents folder is probably located at %UserProfile%\Documents.

A recap- Explorer might show your Documents folder in several places, but those are simply shortcuts and not the actual locations of the folder. Regardless of where you find "Documents", clicking on it will take you to the same place.