There are no lines around shared borders. Any lines you see in Preview
or Preview in Browser are part of the page content or styling. Lines
that are *only* seen in Design view will not appear in the finished page
when viewed in a browser.
Those dotted lines (which, I assume, is what you're referring to) won't
appear in the final page when viewed online in a browser - they are "design
time" to give a visual indication as to what area of the screen is the
shared border/s.
Otherwise,can you give us a link to the page/s that this is occuring on?