You spawn the hitbox in the centre of the player (image point 0) so possibly the enemy will hit the player first. You can set up a new image point for the player in the image editor, then on the spawn action you have there you can see the image point 0 you can change it to image point 1, this is for spawning the hitbox elsewhere like on the player hand.
Where the enemy can collide with the hitbox and the player, well that's down to your own game design. You can adjust the collision box of the player and the hitbox separately. The issue about pressing the attack button twice is probably that the hitbox misses the enemy. The enemy may have a smaller hitbox than you think and can be edited in the image editor.