My website is positioned well on Google(organic search.
I want to create a webpage that targets searchers in a different city.
I realize I would need to write new content for this landing page so it is not simply duplicated content for the key phrase, but is just one page targetting the new city enough to for google seo purposes? (e.g. domainname/new-city.htm
Really depends on the subject, the city and the content. Why not include the new city you are targeting within your existing site rather than making a 1 page entry page. After all you are directing them to the same content on your main site.