New Tab - CMD-T OSX 10.10.3
Not sure if it's just me or everyone, but when I use CMD-T to open a new tab, it's trigger two new tabs to open instead of one on snapshot 184.108.40.206 (Developer Build). I tried searching to see if anyone else posted about it, but nothing showed up.
Interesting. I quit and re-opened Vivaldi and it stopped doing it. Guess it's just a bug that surfaces on occasion.
Confirmed on my end. It's gone after a browser restart. But after a few minutes of browsing around the bug re-appears.
I suggest you report it via the Bug Wizard. Even if it's temporary, a bug is still a bug.