I take 100mg of lamictal daily, (50mg 9am, 50mg 9pm) i accidentally took an extra centrum daily multivitamin without realizing it, literally has only happened once(i.e today).... do i need to worry about serious side effects?

Hi, if it's just once, it is very unlikely to affect your lamictal level significantly or cause vitamin overdose.

Should not cause any substantial interaction with your Lamictal, and not clear to me why you are taking multivitamin prep, as this approach tends to be costly without any significant benefit, if you ingest a balanced diet in the USA.