This has happened to me ever since the features got added to 2.8.
I assumed that I wasn’t the only one having this problem but I haven’t seen anyone else talk about it.
When I add the HDRI or matcap images they are not duplicated but if I close Blender and open it up again every preview is duplicated.
I am on Windows10 x64 with an AMD Radeon R9 390X with the latest drivers.
Hello, NinthJake,
Same problem for me: matcaps and HDRIs are displayed twice when I click on their lists.
However, they are only displayed once in the User Preferences.
They are only present once in the 2.8/datafiles/studiolights/matcap (and world) folder.
Same behavior if I manually add a matcap or HDRI in the folder: it is displayed twice.
Have you figured out how to solve the problem?
Unfortunately I haven’t found a fix for it, I kinda just learned to live with it 
But now with the beta rapidly approaching I think I’m gonna file an official bug report. It is probably a really easy fix.
Just clear your preferences folder…
I have tried that. It doesn’t help.
There’s got to be duplicates somewhere. The default matcaps shouldn’t show in user preferences, only the ones you add. Have you tried renaming the matcaps folder to see what would happen? Looked in AppData folder?
Go there:
%AppData%\Blender Foundation\Blender\
and delete the 2.8 folder
I don’t have a folder in AppData. I’ve only ever used local Blender versions with their own config folders.
I did however delete the entire Blender 2.8 folder and replace it with a fresh build. Granted I don’t have duplicates any more but now Blender 2.8 throws me an error when I try to open the light user preferences.
Traceback (most recent call last):
File "D:\Programs\Blender\Blender 2.8a2\2.80\scripts\startup\bl_operators\wm.py", line 2515, in execute
bpy.ops.screen.userpref_show('INVOKE_DEFAULT')
File "D:\Programs\Blender\Blender 2.8a2\2.80\scripts\modules\bpy\ops.py", line 198, in __call__
ret = op_call(self.idname_py(), C_dict, kw, C_exec, C_undo)
AttributeError: Calling operator "bpy.ops.screen.userpref_show" error, could not be found
location: <unknown location>:-1
EDIT: Figured out the problem.
I’ve discovered that the duplication only happens if you have a config folder inside of your Blender 2.8 folder. I’ll go ahead and report it as a bug now.
Good work!
Same for me: without the config folder, no duplicates !
With an empty config folder, maps are duplicated again
The problem seems to have been solved since this morning. 
Awesome. Time to download a new build then 