good phrasesI've seen words like these:
Do they mean the same as 平坦(的), 明白(的) and 开心(的)??
If yes, what is the point of using the doubled ones?
This is called reduplication of verbs (明白) or adjectives (平坦, 开心) in Mandarin Chinese.
The function of adjective reduplication is to intensify/denote emphasis, to 'soften the meaning of the words' or describe something or someone in a less acute degree, or to make them less direct.
Verb reduplication is typically used in imperative expressions, in which it lessens the degree of imperativity, or used to convey the meaning of informal and temporary character of the action.
Hope this helps.