I was talking with my wife, a native speaker of Mandarin, about the characters 複 and 復 and how these two characters were merged into 復 in Mandarin. In writing a paper in simplified characters, she had trouble choosing the second character of chong2fu4, whether 複 or 復. I, native speaker of Japanese, also had trouble remembering which one. It turned out that the word is written as 重複 not 重復. Both characters 複 and 復 have almost the same meaning. Particularly, in this combination, "multiple" or "repeat". We mistakenly thought it should be 復, since the word means "to repeat."
Curiously, two characters in writing developed despite the fact that Old Chinese had only one word in speech. In Old Chinese, the two characters were pronounced the same. 儘 and 盡 or 着 and 著 had different pronunciations in old Chinese. So, it is no mystery that ancient Chinese invented these characters (儘, 盡, 着, and 著) to codify the separate words in speech. But, when it comes to 複 and 復, ancient Chinese people seem to have invented these two characters out of one word to highlight the subtle differences in the meaning of the word.
What an ingenuity (or a nightmare to those who intends to learn the language)!