That code looks alright. The comments go through and get posted, the second slash is being inserted by the browser, the error starts before the comment form, with a wrong link TO the comment form
Note the space before #comments
Both the (main) links to posts (via the post headlines) and the direct links to the comment area of posts have a trailing space.
There appears to be a trailing space somewhere in your settings, see if there's a trailing space at Site Admin -> Settings -> Permalinks -> Custom Structure
Click into the right edge of the "Custom Structure" text field and see where the cursor ends up at. If there's a trailing space, delete it, click "Save changes" and try posting a comment again.