难分难解 hard to separate
Explanation
形容双方争斗、比赛等势均力敌，难以分出胜负；也形容关系非常亲密，难以分开。
Describes a situation where both sides are evenly matched and it's difficult to determine a winner; also describes a very close relationship that is difficult to separate.
Origin Story
话说古代两大名将，赵云和马超，在战场上狭路相逢。二人武艺超群，兵器碰撞之声震耳欲聋，刀光剑影，杀得难分难解。赵云的长枪如龙，马超的宝剑似虎，招招致命，双方你来我往，斗了个昏天黑地。一时间，胜负难料，周围的士兵们都看得目瞪口呆。战斗持续了很久，天色渐晚，两人仍旧难分难解，最终在双方将士的劝说下，才罢兵休战，各自退兵。这场战斗，不仅展现了两位将军的非凡武艺，更成为了历史上一个难分难解的经典战例。
In ancient times, two great generals, Zhao Yun and Ma Chao, met on the battlefield. Both were masters of martial arts, and the clash of their weapons was deafening. Swords and spears flew, and the battle was fiercely fought, with neither side able to gain a clear advantage. Zhao Yun's long spear was like a dragon, and Ma Chao's sword was like a tiger, each strike deadly. The two fought relentlessly, until the sun began to set. They were still locked in a stalemate when their troops urged them to stop, and they retreated. This battle not only showcased the exceptional martial arts skills of both generals but also became a classic example of an irresolute battle in history.
Usage
常用来形容双方实力相当，竞争激烈，难以分出胜负；也用来形容感情深厚，难以分开。
Often used to describe that both sides are equally strong, the competition is intense, and it is difficult to determine a winner; it is also used to describe a deep bond that is difficult to separate.
Examples
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这场比赛，双方实力相当，难分难解。
zhè chǎng bǐsài, shuāngfāng shí lì xiāngdāng, nán fēn nán jiě
In this match, both sides were evenly matched and it was hard to tell who would win.
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他们俩从小一起长大，感情难分难解。
tāmen liǎ cóng xiǎo yīqǐ zhǎng dà, gǎnqíng nán fēn nán jiě
The two of them grew up together, and their relationship is inseparable.