你选择了和我们完全一样的壁纸——真是巧合！

You chose exactly the same wallpaper as us - what a ___coincidence!

纯属巧合，五十年后我又遇到了我的老朋友和同学。

Just by ___coincidence I met my old friend and classmate again fifty years later.

我们相遇纯属巧合。

It was pure ___coincidence that we met.