How to open a Finder window even if one is open on a different space already, in Terminal?
You might use a script like this:

#!/usr/bin/osascript

tell application "Finder"
    make new Finder window to POSIX file (system attribute "PWD")
    activate
end tell


The script above always creates a new window. This wouldn't create a new window if a window with the same name already exists on the current space:

#!/usr/bin/osascript

launch application "Finder"
set pwd to system attribute "PWD"
set text item delimiters to "/"
tell application "System Events" to tell process "Finder"
    set w to windows where name is text item -1 of pwd
    if w is {} then
        tell application "Finder" to make new Finder window to POSIX file pwd
    else
        perform action "AXRaise" of item 1 of w
    end if
    set frontmost to true
end tell


System Events only includes windows on the current space.