How Long Does Next Choice Emergency Contraceptive Provide Protection?

Hello. I have a question about Next Choice emergency contraceptive. I took the pill 4 days ago after having sex with my boyfriend. (we had used a condom also.) Last night we had sex (unprotected) and I was wondering if it is safe to take the emergency contraceptive again tonight? Is the first one that I took still effective? or do I have to take it again?