Why did I hit a golf ball twice in one swing?

I was at the range today, and I had an exceptionally good day. For the most part, everything I was hitting was great, except a couple of time I hit the ball twice on my iron swing, and it went 30 or 40 yards and flared to the left. I knew I hit it twice because of the way it felt; like i hit it once with the club at the normal impact position and again with my club extended when it is on the same plane as my knees. I guess this has to happen to you in order for you to understand it? Anybody have any ideas as to why this happened?