Lucalaz,
As 737OPR said, ignore what he wrote because it was incorrect, of course the autothrottle will move the thrust levers in manual flight.
The machine is intelligent enough to realise the difference between a normal G/A and the windshear manoeuvre, not least because it's gone to great trouble to produce a great big 'WINDSHEAR' on your TV for you. Therefore it is only necessary to push the TOGA switch once. If you do it twice you are not going to fall out of the sky but it will make no difference to the thrust you get. I am guessing that Boeing consider the autothrottle application of max thrust as being aggressive enough to not need to write anything extra about it.
I find it slightly more interesting that in manual flight the autothrottle is disengaged once having applied max thrust, whereas in auto flight it is left engaged. I can understand the reason for disengaging it (stops the thrust changing for some reason when you want it to stay put at max), so why don't we disengage the A/T once max thrust is set in auto flight?
Answer to your question though; push TOGA as many times as you like but you only need to push it once for the windshear manoeuvre case.