late answers . but here you gp -
Tidying registry will have no effect on you PC speed unless it is completely corrupted - but you would know that as PC would not be working.
... Registry is just a simple text list really, so does not use much space. Way back when PC's were really slow (1990's) a registry tidy helped a bit.
.. If you want to delete them all off - just Create a System Restore point .... if you don't know what that is ggl it, and don't mess with registry if don't know what it is telling you in the CCleaner list.
Duplicates - depends how much space they are taking - If they are your own personal files that you have managed to save and copy double etc - just keep your own personal file / directory tidy with file explorer not ccleaner - this is no different to keeping a physical cupboard or filing cabinet tidy...
... If the duplicates look like strange system type names, leave them alone unless you know what you are doing.
... I NEVER use deletion of duplicate files from a software like Cleaner - too easy to delete something you didn't understand or didn't want to delete.
... . look under C:\Users\<username>\Downloads ... you may well have downloaded stuff several times by clicking in internet. sort that out by deleting what you don't need. Download directory is for downloads, not for permanently storing stuff.