I think we just have a small misunderstanding here. As PAULW says (rather loudly) a second spraying is not worthwhile if one has just turned the soil over.
One would have to be prepared to wait until weeds reappeared and were a reasonable size for a second dose to be worthwhile and would probably therefore miss the planting window for several mainstream crops.
GazFoz, may I suggest you turn the ground over with a fork, pulling out any obvious weed roots as you go, then cover with weed suppressant fabric (the type that lets water through) and plant the spuds through it ? The spuds will themselves help keep the weeds down.
For more detail on this method of spud growing you'll need a more informed brain than mine.