In my experience, the Goto Definition command/function (available either through the Hover over or through the “Goto” menu and the keybinding for that command) … will always open a new buffer for the location of the definition … even when that file is already open in another group.
I like my groups and am comfortable manipulating/navigating them (thanks mostly to the Origami package … and I often have my files organised in a logical manner such that a definition is likely to be in a file in another group from its use/call/reference.
Having a definition open in an already buffer would be much more useful, or at least having some control over it.
I haven’t found a way to handle this. The LSP plugin seems to have a means of managing this (see this GitHub Issue #1994. Anyone have any thoughts?