Jumping player

I want my player to jump once then land, but if I press the jump button again, the player will still jump in midair(while falling). Its a 2d, c# game.

void Start () {
}

void Update () {
		if (Input.GetKeyDown ("space")){
 transform.Translate(Vector3.up * 260 * Time.deltaTime, Space.World);}

	}

A simple boolean will help control it:

// at the top with your class members
bool isJumping = false;

// then in your update function
Void Update()
{
    if (Input.GetKeyDown("space") && !isJumping)
    {
        isJumping = true;
        transform.Translate(Vector3.up * 260 * Time.deltaTime, Space.World);
    }
}

Then you just set isJumping back to false somewhere (not in update as I believe it runs many many times per second).
Beware this is untested as I am at work. Hope this helps.