Is there a shortcut key to duplicate a tab in Google Chrome?

You can just press Alt+D then Alt+Enter. You don't even need to let go of the Alt key.

There is not currently a keyboard shortcut to duplicate a tab in Google Chrome (see a list of keyboard shortcuts). However, it looks like Chrome may soon enable support for extensions to duplicate tabs, which means you might soon be able to find a Chrome extension that can duplicate a tab through a keyboard shortcut. Currently, all of the extensions that show up in the store don't preserve history.

Non-keyboard-wise, there are two ways you can duplicate a tab: either middle-click on the refresh button, or right-click on a tab and choose "Duplicate" from the menu:

The Duplicate Tab Shortcut Key extension allows native duplicate tab functionality with a customizable shortcut key.

Using the extension allows you to duplicate a tab with the browser's built in functionality so the history is preserved and the page isn't reloaded.

The tab key is customizable using Chrome's API - the extension doesn't just monitor each tab's DOM for the shortcut key event as some of the other extensions do.

The extension comes with fairly thorough documentation.