The only thing I retained from primary school recorder classes was the “du” sound! So I mainly use that. I’m working on double tounguing with “duh guh” at the moment. It took a while to get it to work faster than single tonguing. I’m using it for some passages which probably don’t strictly need it, just for the practice. I’ve looked ahead at some of the more specialised double tonguing patternss in Alan Davis’ “Treble Recorder Technique”- can’t do them yet, but I can definitely see the need for them.