(Ni no mai wo enjiru; “to dance the second dance”)
Committing someone else’s previous errors, even with the awareness of what has happened. Seeing someone fail, and then making the same mistakes.
This simple phrase begins three characters in with the noun 舞 (mai), “dance.” The associative particle の (no) connects it to, and modifies it with, the number 二 (ni), “two.” The resulting noun phrase is marked as the object of a verb by the particle を (wo), and the verb in question is 演じる (enjiru), “to perform,” in conclusive form.
This saying is based on a gagaku (classical court music) performance in which a dance called the Ama (安摩) was followed by a second dance (the 二の舞) that followed the form of the first but with comedically exaggerated errors. (The curious can watch a nearly 30-minute performance here; it’s probably very different from what you expected!) The saying used to simply refer to a “repeat performance,” but over time greater emphasis has been placed on the making of mistakes.
Original* verb form 演ずる (enzuru) is also perfectly acceptable, although apparently less common (*In the sense that the ～じる structure is derived from the ～ずる structure as a way to turn nouns into verbs.)
Another expression that may appear almost identical at first glance is 二の足を踏む (ni no ashi wo fumu), literally “to take a second step,” but in use meaning “hesitation.” Mixing the two up and writing this “dance” saying as 二の舞を踏む (ni no mai wo fumu, where 踏む is “to (take a) step,”) is technically an error, but has been a common enough occurrence that it may pass unremarked-on. However, replacing 演じる with 繰り返す (kurikaesu), “repeat,” is considered an error.
(“Junkesshou de Sunada ni sente wo utarete sugu ni makete shimatta senpai no ni no mai wo enjitakunai kara, ashita no shiai ni sonaete aitsu no waza wo shikkari kengaku shite haaku shite okou to omou.”)
[“I don’t want to just repeat the way sempai immediately lost the initiative, and the match, to Sunada, so I’d like to prepare for tomorrow’s match by watching his technique until I really get how it works.”]