A subclip is simply a section of the full clip. Set by in/out points, and it shares the Master Clip of the original full clip. The Master Clip data is what Premiere stores on that clip, its properties, audio settings, uses, and of course any effects applied to the Master Clip tab of that clip used on any sequence. Which will be applied to all other users (including subclips) of that clip on other sequences.
A duplicate clip is a separate listing of the clip in the Premiere database, and therefore has a separate Master Clip reference.
So you can apply different effects in the Master Clip tab of a duplicated clip without affecting the other instance of that clip in the project. And set the properties for that clip differently if you wish.
This is especially useful for applying different things to the complex media such as many RAW and Log formats where their manufacturers provide for options to modify the clip via the Master Clip tab on a sequence. For my BlackMagic RAW files, that includes ISO, color science, and many other things.
In a single use of the clip in the project, anything I set in the MasterClip controls for the view of that image apply to all uses of the clip.
If I want a different look to a section of that clip, I can simply "duplicate" the clip in the project panel, and apply different settings in the Master Clip controls for BMRAW without affecting the use of the clip from the other instance.
Neil