Is diverticular bleeding more common in people who already have diverticulosis?

Of course. You can't bleed from a diverticulum if you don't have any. Diverticulum bleeding occurs on people who have diverticulosis.
Yes. By definition, diverticular bleeding occurs in people who have diverticuli. This increases in frequency with age and is more common on the right side of the colon rather than the left (in contrast, diverticulitis, which occurs due to rupture of one of these diverticuli, usually occurs on the left side).