As a double major if you do reasonably well in only the second major will it affect your chances of getting into a good grad school?
If your second major (economics) is not related to the graduate program (pure? math), your performance in the second major will be viewed the same way as your performance in elective courses.  It is not that important but it will count towards any GPA cutoff.
If you were applying for a graduate program in economics with an intent to do quantitative research, bad math grades would be more important because math is used in some economics.
I would not recommend retaking a specialized course you do not plan to use for anything unless you would enjoy it.  It would be more efficient to take a course that is useful and also increases your GPA.