100% repeatable. Game v1.80143869 with no mods loaded.
With a backpack on your back, taking a backpack from a full (or near full) vehicle or crate results in a bugged duplicate backpack (SP and MP). If another player picks up the bugged duplicate backpack, both players see glitchy jumping backpacks. (MP) If original player then drops backpack, both players see 2nd player model stretch into bizarre pattern. (MP)
I consider this a major issue because in my MP mission, I constantly have full or near full trucks (which routinely results in this bug for my players).