So I have scheduled posts 5 weeks in advance 5 days a week at 8 pm...then I thought it would be easier to create a custom queue. So I created 5 days a week at 8 pm... If I then create a post and say 'add it to the next slot' it schedules it for that day at 8 pm. 

the custom queue should recognise that I already have a post going out at 8 pm that day and follow on from 5 weeks later. Otherwise the whole...."so you don't double up on posts" doesn't work unless you start from scratch.