Should I cite a concept I have already cited earlier?
This is hard to answer precisely without seeing your entire document.  In general, I'd say if it is very obvious what you are talking about, then you do not need to cite again.  The problem is that what is "obvious" to one person is not obvious to another.  Especially it can be hard for you to know if it is obvious or not because you are too close to it.  One suggest is to have a friend or other graduate student read the whole paper, and then ask them for an opinion.  Or, just err on the side of slightly more citations.