Dual vs Duel
Dual is an adjective that means double; twofold.
A duel is a prearranged fight between two people armed with deadly weapons.
Duly vs Dully
Duly is an adverb that means in due manner; at the right time or in the proper way.
Dully is the adverb form of the word dull that means physically or mentally slow; not sharp; not vivid; boring.
Delve vs Divulge
Delve is a verb that means to dig or search.