The word "噶噶" is formed by duplicating the single character "噶", which serves as a phonetic unit to approximate a sound. This reduplication creates an onomatopoeic effect, where the repeated syllable directly mimics a repetitive noise, such as a duck's quack or a similar vocalization. The formation relies solely on the character's sound-representing function, without inherent meaning, to produce a word that imitates auditory phenomena.