adverb
each person saying; all saying; unanimously; in unison
Describes a situation where many people say the same thing, either together or one after another. Often used with verbs of speaking like 言う, 叫ぶ, 褒める, etc.
みんなが口々に彼の成功を褒めた。
Everyone praised his success, one after another.
参加者は口々に反対意見を述べた。
The participants voiced their objections, each in turn.
Means 'in chorus' or 'unanimously', literally 'aligning mouths'. It implies a coordinated, simultaneous expression, whereas 口々に can describe sequential individual remarks.
Reduplication of 口 (mouth) with the adverbial particle に, literally 'mouth by mouth'. The iteration mark 々 indicates repetition of the preceding kanji.