I use await MainActor.run { main thread stuff } instead of DispatchQueue.main.async when I am in an async func. Is this comparable to Task { @MainActor in main thread stuff }, which is really new to me, or are there differences?

Are you storing your project files in iCloud Drive maybe? And use Xcode also on another Mac? That may be the cause of duplicate files with suffix 2, iCloud not working 100% all of the time?