Here's the negative top-margin

Input{margin-top:-.1em;}

Here's the negative bottom-margin

Input{margin-bottom:-.1em;}

This proves it's the top border being repeated, because I disabled it and the bug doesn't show up.

Fieldset{border-top:none;} Input{margin-bottom:-.1em;}

This fieldset has no legend, which cures the bug.

Input{margin-top:-.1em;}