Most recently, two S&W Shield's in 9mm. Bought one for the wife, and liked hers so much that I went and bought one for me the next day. Lucked up and got consecutive serial numbers as well.
I'm more likely to get twins in out of production firearms if for no other reason than spare parts. I don't have one, but I don't think you can ever get enough S&W K-22's. Those old guns are just superb. I'm not sure you could buy anything new that was better.