Handle multiple tabs using the same URL

[AhsanAyaz]

Problem

At the moment, if you have multiple tabs using the same URL, the extension treats them as multiple pages.
This could result in some discrepancies which could be avoided.

Solution

If there are multiple tabs with the same URL, use the first one of them only.

[LaylaSilbernberg]

I'll work on this one!

[AhsanAyaz]

Fixed by #9