wait... you used mirror geometry for that?

i just tried to duplicate what you've done to see if i could find out an answer

if you used mirror geometry then it's expected to result like that, because mirror geometry only mirrors in the x y or z axis (i think)

it appears, at least on my end, that you can do this if you play with the pivot point and use duplicate special and play around with which diretion its duplicating in then you'll get what you're wanting

and just combine and merge verticies if need be