Sidearm/binocular duplication:
Find a dead body with a pair of binoculars or a sidearm.
Add sidearm or binoculars to your hands, clothes inventory, or vest inventory (backpacks not checked).
Drop acquired handguns or binoculars in another location, repeat.
Clothing duplication:
Find a dead body with clothes while wearing your own clothes.
Drag body's clothes to your clothes slot.
Your starting clothes should have been deleted and the body should remain wearing clothes.