I think I can help. In this situation, just work with your file in Devonthink like you would work with it in the Finder. I would delete the copy outside of Devonthink, but do some experimenting to see how that all works.
Unless you have reason not to, import everything you work with. Don’t import your Documents folder or anything another app manages. Like Dropbox, for example.
As others have said, go slowly. It doesn’t take more than an afternoon to get a black belt in Devonthink. Then, of course, you’ll keep finding new features and new ways to work with Devonthink.
Also remember that if Devonthink is doing something that seems complex, it generally means there’s a better way to look at what DT is doing.
In the case of a file you drag into Devonthink (importing, not indexing it), you either copied it to Devonthink (simple drag and drop) or moved it (Command-drag and drop). If you moved it, there’s one copy, the one in Devonthink. If you copied it, you have two independent copies of the file that can be independently edited, one you see in Devonthink, the other in your Finder.
In the case of imported files, Devonthink finds a nice spot for that file inside the Devonthink database - but it’s really simpler than that.
A Devonthink database is a “package”. That’s a regular folder on your disk, but the Finder will show it as one monolithic object. You can see what’s inside by using the Finders Show Package Contents feature, but there’s no need to do that. Render unto Devonthink what is Devonthink’s and let it have sole control over what’s in the Package. Just rest assured your files aren’t consumed by some Borg-like database. They remain viable files in their original form. Storing a file in Devonthink is as safe as storing it with your Finder.
Devonthink stores files in folders inside the package according to its own rules. The groups you see in Devonthink amount to a curated inventory. When you move files from one group to another in Devonthink, they don’t move on disk, they are just displayed in the new group.
Replicating a file flags it to appear in more than one place in Devonthink’s groups. Edit either appearance to modify the single file represented.
Duplicating actually creates another copy of the file.
There are many more features, of course, but just keep in mind that Devonthink is consistent in how it does things and keep exploring.