You might find this useful..
This is sort of what I use for any left/right movement and it works great.. just needs a single number instance variable.
+for each player
-> set player.moving to 0
+Left arrow down
-> subtract 1 from player.moving
+Right arrow down
-> add 1 to player.moving
+player.moving not equal to 0
-> player apply force of player.speed*player.moving
so if neither (or both!) of the keys are pressed the player doesn't move
Player.moving becomes -1 when the left key is pressed. This ends up negating the player.speed.. causing it to go left instead of right.
*edit: and i like this method also because the -1,0,1 variable can be useful elsewhere..
for example, if you wanted to tilt the character sprite towards it's direction.. it's just one event:
-> set angle to 15*player.moving
btw, 
Nimtrix, you could also just place the 'every tick set false' event at the very top
one last thing: you don't need "player: is keys" there for every single event, just make it the top level event, with everything else a sub-event of it.
Oh, I've never used sub-events before - sounds like I should be!
Right well, I'm about to try these suggestions out I'll let you know how I get on.
Thanks!