Close New Duplicate Tabs — Sunfur

This open-source extension helps you to stop accidentally opening the same webpage twice and cluttering your tabs bar.
It automatically detects and closes a newly created tab if its URL is already open in another tab, reducing memory usage and keeping your tab bar clean.

Unlike other extensions with similar functionality, this one works on newly created tabs, meaning that you wont lose tabs with history. It still enables you to close all duplicate tabs through the context menu if you have some left over, though.

This extensions behaviour can also be customized extensively in the extension options - if you want it to, it can even ignore search strings in URLs for duplicate checking or redirect your focus when a duplicate gets closed.