At once and Headlong

At once adverb - Without delay or hesitation; with no time intervening.

Headlong and at once are semantically related. in fast topic. In some cases you can use "Headlong" instead an adverb phrase "At once".

Headlong adverb - With excessive or careless speed.

At once and headlong are semantically related. Sometimes you can use "At once" instead an adverb "Headlong".