So I managed to get it straight on the edges, but when I smooth it, it still treats it as two separate objects and ends up with a giant hole in the middle ok im not an expert however i can suggest you use Mirror geometry and make sure you tick merge with original and that would solve the issue , however i think the problem with the duplicate special is that you will have to combine the two halves and afterward select the vertices in the middle and "Merge" im sorry this what i know hope you get it solved buddy cheers