@pesala it is already the default behaviour, in Settings, Bookmarks, to disable:
Open Bookmarks in New Tab
Disabling this option causes only opening bookmarks in the current tab, no matter if it is start page or some website. This way you can open a bookmark in the current tab with the site already open, so you replace her. But the point of this topic is to replace only start pages that are not used.
The subject of this feature request concerns the use of empty start pages for open page by double-clicking (rather not the middle click used to open pages in the background), regardless of whether it is a bookmark, history entry, link from notes, etc.
It is only for discussion which start pages the browser should use, if there are more than one already open, and in what order (e.g. from left to right, only the most right etc), these can also be as the options.