Some edits to Dolphin source code to allow single instance. Just add the files in the src folder of Dolphin and add dolphinsinglewindow.cpp to CMakeList DOLPHINSRC and you are ready to go.
[obsolate] A better and more complete solution by intika here: https://github.com/intika/kde-dolphin-single-instance .
[final] The feature was intorduced to KDE by feverfew here: https://phabricator.kde.org/D16648 .