Duplicate content is duplicate content. Having said that, if you were able to remove the development site from public view quickly, it probably wasn't around long enough to affect your main site's rankings. Additionally, your main site would probably not have been affected since Google most likely would have considered it the primary source of the information and would have penalized the development site as the duplicate.
One good way to guarantee this isn't an issue in the future, besides making sure your development site stays private, is to use the rel=canonical tag on your development site and make sure it points to the main website. Then if it is accidentally exposed again it will tell Google that the other site is the original source and to disregard those pages.