As I know, we could use “for” with perfect tense to talk about a period of time. However, I am still not sure how to use “for”.
Could you tell me which sentence below is okay? I don't know I should use only one "for" or two "for"."
I have been waiting for a bus 20 minutes.
I have been waiting for a bus for 20 minutes.
Both sentences are acceptable. Both, however, are a little awkward.
The first makes use of the quite ordinary expression
I have been waiting 20 minutes.
For is not necessary; the semantics of the verb wait make it perfectly clear what role 20 minutes plays in the sentence. But when you put for a bus between the verb and the time expression, the connection is broken; it takes a few microseconds of extra mental processing to figure out what role 20 minutes plays.
The second runs you up against what linguists call horror aequi—we don’t like using similar constructions too close to each other. For X for Y just feels uncomfortable. (This is a cross-language phenomenon and is probably what raised your question in the first place.)
The solution is very simple—flip your two adjunct phrases:
I have been waiting 20 minutes for the bus.
