Help: Doubling up on RHOGAM??

I miscarried last year and have a negative blood type so I got a RHOGAM shot effective thru 9/4/16 (18 months). My dr has my shot scheduled for tomorrow because I will be 28 weeks with a new pregnancy, the nurse said it was fine if they over lapped by a couple days but the more I think about it I feel weird, like will it counter act or complicate things? I can't find anything on the Internet. Hoping someone else went thru something similar or maybe a nurse or someone knowledgeable can help. Wondering if I should just reschedule for 9/4 so I dont stress about it.