private bool isTurned = false;

void Update()
{
    if (Input.GetKeyDown(KeyCode.A))
    {
        if (!isTurned)
        {
            transform.Rotate(0, 180, 0);
            isTurned = true;
        }
    }

    if (Input.GetKeyUp(KeyCode.A))
    {
        if (isTurned)
        {
            transform.Rotate(0, 180, 0);
            isTurned = false;
        }
    }
}