If you're a workspace member with an Editor or higher role, you can make a copy of a project in the workspace. Hover over a project's thumbnail and select the "Make a Copy…" option under the settings menu.
The Zeplin Summer Breeze release has a new options button near the project name where you can manage your project, including making a copy of it.
☝️ Only the project owner can copy a project on the Free, Basic, and deprecated plans — Starter and Growing Business plans.
You can select whether to include users, comments, versions, and flows in the copy. The duplicated project will automatically include all screens and resources in the local styleguide.
💁♀️ If a project is linked to the styleguide, the duplicated project will be linked to that styleguide, too.