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二度あることは三度ある
(Nido aru koto wa sando aru;
“Something that happens twice, happens three times”)

Definition:

Things tend to repeat. If something has happened before, especially more than once, then it should be assumed that it can and will happen again. An admonition to be careful not to repeat mistakes.

Breakdown:

The whole phrase revolves around the noun こと (koto), “(abstract) thing,” as shown by the topic-marker particle は (wa). The noun is modified by verb ある (aru), “to exist,” in prenominal form, and the details of its existence are number-noun 二度 (nido), “two times.” The comment following the topic marker, meanwhile, is number-noun 三度 (sando), “three times,” in turn modifying the verb ある (aru), which appears this time in sentence-final form.

Notes:

This phrase seems to have originated in a jōruri piece called 『驪山比翼塚』 (Meguro hiyokudzuka), a likely-fictional place name.

Example sentence:

「昨日も今日も学校に自転車に乗って行って、事故に遭ったんじゃない？二度あることは三度あるというから、明日はもっと気を付けた方がいいよ」

(“Kinou mo kyou mo gakkou ni jitensha ni notte itte, jiko ni attan ja nai? Nido aru koto wa sando aru to iu kara, ashita wa motto ki wo tsuketa hou ga ii yo.”)

[“You rode your bike to school both yesterday and today and got into accidents, right? If something happens twice it can happen a third time, so you should be more careful tomorrow.”]