It can be dangerous to "double up" on multivitamins if you do it for a long time. This is because you might then be taking in too much Vitamin A & Vitamin D. Most multis are pretty close tothe recommended daily dosage so doubling it for weeks or months could be a NOT good idea.

Instead, supplement the "holes" in the multi with other nutrients that you feel you need.

Vitamin C & the B's are rather self-regulating. Because they are water soluble, if you take in more than your body needs, they just run out the other end.