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English Translation

to contradict oneself, self-contradictory

Breaking it down

1. 自相: (doing something) to members of one's own group
2. 矛盾: lit. pike and shield, fig. conflict, contradiction

Source: Google Image Search

Story in Chinese

　楚国有一个卖兵器的人，到市场上去卖矛和盾。
　好多人都来看，他就举起他的盾，向大家夸口说：“我的盾，是世界上最最坚固的，无论怎样锋利尖锐的东西也不能刺穿它!”
　接着，这个卖兵器的人又拿起一支矛，大言不惭地夸起来：“我的矛，是世界上最尖利的，无论怎样牢固坚实的东西也挡不住它一戳，只要一碰上，嘿嘿，马上就会被它刺穿!” 他十分得意，便又大声吆喝起来：“快来看呀，快来买呀，世界上最最坚固的盾和最最锋利的矛!”
　这时，一个看客上前拿起一支矛，又拿起一面盾牌问道：“如果用这矛去戳这盾，会怎样呢?” “这——” 围观的人先都一楞，突然爆发出一阵大笑，便都散了。
　那个卖兵器的人，灰溜溜地扛着矛和盾走了。

Source: http://chengyu.t086.com/gushi/421.html

Story in English

In the state of Chu there was a man who sold weapons, and who went to the market to sell his spears and shields.
Many people came to look at them, and he raised his shield and said to everyone: "My shild is the strongest in the world, no matter how sharp a pike is, it cannot go through!"
Then the weapon seller picked up a spear and boasted: "My spear is the sharpest in the world. No matter how strong and solid a shield is, it cannot stop my spear! If the shield just touches it, then it will be immediately pierced through." He was very proud, and shouted again: "Come and look, come and buy the strongest shield and sharpest spear in the world!"
At that moment, a spectator took up a spear and also a shield and asked: "If I pierced this shield with that spear, then what would happen?"
"This --" stuttered the weapon seller.
The other spectators first were all shocked, then suddenly bursted out in laughter, and then went away.
The weapon seller took up his spears and shields gloomily and walked away.

Translation by Celine Li

Usage

他刚刚说去看电影了，现在又说今晚一直待在家，显然是自相矛盾。