X是X，Y是Y: keep two things apart
【四70】口语格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Repeating the same word on both sides of 是, X是X，Y是Y keeps two things firmly apart: 朋友是朋友，账是账. English: “business is business”, “a deal's a deal”, “yesterday is yesterday, today is today”.
It is used to refuse a mix-up (工作是工作，生活是生活: don't mix them) or to accept a difference (他是他，我是我). Both halves are complete and symmetrical, with no 也 or 都; a practical conclusion often follows (……你得交作业).
Versus 不是……而是……: that denies A to assert B; X是X，Y是Y keeps both and only refuses to blend them. Don't confuse it with 好是好，就是太贵了 (concession, “it's nice, but…”), another use of the same shape. Spoken register.
Examples
朋友是朋友，账是账，我们还是分开付吧。
Péngyou shì péngyou, zhàng shì zhàng, wǒmen háishi fēnkāi fù ba.Friends are friends and bills are bills, so let's split it.
今天是今天，明天是明天，明天的事明天再说。
Jīntiān shì jīntiān, míngtiān shì míngtiān, míngtiān de shì míngtiān zài shuō.Today is today and tomorrow is tomorrow; leave tomorrow's business for tomorrow.
他是他，我是我，你别拿我跟他比。
Tā shì tā, wǒ shì wǒ, nǐ bié ná wǒ gēn tā bǐ.He is he and I am I, so don't compare me with him.
工作是工作，生活是生活，下班以后别谈公司的事。
Gōngzuò shì gōngzuò, shēnghuó shì shēnghuó, xiàbān yǐhòu bié tán gōngsī de shì.Work is work and life is life, so don't talk about the company after hours.
一是一，二是二，这件事要分清楚。
Yī shì yī, èr shì èr, zhè jiàn shì yào fēnqīngchu.One is one and two is two: this matter has to be made clear.
昨天是昨天，今天你得把作业交了。
Zuótiān shì zuótiān, jīntiān nǐ děi bǎ zuòyè jiāo le.Yesterday is yesterday; today you have to hand in your homework.
Common mistakes
✗ 朋友是朋友，账 → ✓ 朋友是朋友，账是账 (both halves are complete).
✗ 他是他，我也是我 → ✓ 他是他，我是我 (no 也: the two halves are contrasted, not added).