Set up a work station to a target you are comfortable with. When you have placed your ball into that station now place another ball approximately 1 foot directly behind the first ball. If you are not comfortable in using another ball, make a pile of sand in the same position, use a wood cover etc. Make a normal swing trying not to hit the ball in the backswing or more importantly on the downswing. Do not try and avoid the second ball by going around it or inside of it, allow the club head to travel over it without hitting it.
If you find you are hitting 2 balls at once you most likely are losing your angle of attack or need to create a steeper angle of attack. Falling back and away is also a sign your angle of attack is not steep enough. Keep swinging without thinking of anything other than missing that 2nd ball. Time for a lesson!