A Market Street lot left vacant by a 2008 fire has been given new life as the site of a two-family duplex built by volunteers for Merrimack Valley Habitat for Humanity. The homes were constructed over an aggressive, one-week building schedule as part of Habitat’s Home Builders Blitz, an annual event that aims to build 200 homes nationwide in five days through partnering with local professionals. The Lawrence effort was led by Howell Custom Building Group, a city business, and included additional contractors from Methuen and Georgetown.