Overlearn in undergraduate College - good or bad idea? I have no choice but to take courses I've already learnt in college.
So is it a good idea to overlearn? 
Would overlearning help me to get into graduate school?

There are many ways for me to spend time:
research, gpa, heavy courseload, or studying stuff beyond my current syllabus

According to my course plan, I'm supposed to start learning Multivariable Calculus in about 5 months and Linear algebra in about 11 months from now. In between now and then is just some humanities courses and a bunch of Physics/Math courses I've already learnt.

How important is a high GPA and to get into a good graduate school?
And why is it important to get into a good graduate school? 
note: I'm a Physics major...
Having a good GPA is important; having a great GPA is not.  Assuming your GPA stays above a certain threshold (about 3.5), other factors like research experience are significantly more important, at least for admission to the best departments.

In particular, if you get a C in a distribution class because you spent "too much" time on that experiment you later published, nobody will even notice the C on your transcript.