Rule about repeating citations
The golden rule of citation is that you attribute everything that you did not invent yourself to the right person(s). 

The golden rule of boring writing is to repeat yourself as often as possible.

Just make sure that it is clear what came from whom. If you say: "In the following paragraph we describe the main ideas of [2]", you do not need to repeat [2] in that paragraph, but if you compare two approaches it might be necessary to mention the papers/people more often.