Synonyms of duffer in English:

bungler, blunderer, incompetent, oaf, dunce, dolt, dunderhead, fool, idiot, booby, stupid person, moron, cretin, imbecile

chump, clot, clod, nitwit, dimwit, airhead, birdbrain, lamebrain, pea-brain, numbskull, thickhead, fathead, blockhead, bonehead, meathead, chucklehead, knucklehead, pinhead, wooden-head, dipstick, dumb-bell, dumbhead, dumbo, dum-dum, noodle, donkey, ass, nerd

British berk, divvy, wally, numpty, wazzock, nit, mug, prat, twerp, twonk, pillock, muppet

Scottish balloon, galoot, cuddy, nyaff

North American doofus, goofball, goof, putz, bozo, boob, lamer, chowderhead, meatball, lummox, dummy, turkey, clunk, ding-a-ling, palooka

NZ Australian galah, drongo, alec, dingbat

British vulgar slang knobhead

North American vulgar slang asshat