又 vs 再

又 is used in declarative sentences and describes the simple repetition of actions. This repeated action has already occurred once in the past.

S 又VO了

北京昨天下雨了，今天早上又下雨了。

再 is used to express something that has already happened and will happen again.

（S） (想/要)再 v # mw + noun

早上我喝了三杯咖啡，现在我想再喝一杯咖啡。

（S） (想/要)再 v # 次

我去年去了中国，今天寒假我想再去一次。

However, when there is 要，想，or 是，you should only use 又。

Subj. + 又 + 要/想/是 Verb + Obj.

昨天考试了，明天我们又要考试。

早上我喝了三杯咖啡，现在我又想喝一杯。

怎么又是你？(How come it's you again!)

昨天我背（bèi，to recite）课文，今天又是我背课文。

reference

https://resources.allsetlearning.com/chinese/grammar/Comparing_%22zai%22_and_%22you%22