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If you are inviting a rehearsal dinner attendee's SO to the wedding, I think you should invite them to the rehearsal dinner as well. Assuming that the rehearsal dinner will be the day before the wedding or another time that is not on the wedding day, you will be requesting that they come for at least one extra day extra in addition to your wedding day. Inviting that SO to the wedding, but saying "hey, find something else to occupy your time for the rehearsal dinner", especially if they are coming from out of town, seems rude to me. I think it's a good idea that you plan on including all SOs.