Link Dragging to Dock Icon
I've noticed this behavior for quite a few versions of Vivaldi, where a link dragged to the Vivaldi dock icon will result in the link being opened 2–3 times (that is, in 2–3 tabs). It is, as far as I can tell relatively consistent: a link dragged from Safari's address bar will open in two tabs, but links dragged from a twitter client will open in three tabs. Links dragged from vivaldi itself to the dock icon also open in three tabs.
Links dragged directly to the address bar or the content area of the browser open in the selected tab, as expected. Similarly, links dragged directly to the tab bar open in a [single] new tab.
I am presently using:
Vivaldi 1.10.834.9 (Official Build) (64-bit)
OS Mac OS X (10.11.6)
Can confirm this. Seems to be a bug. You can report it here -- https://vivaldi.com/bugreport/