Yes, I usually set up at least 10 easels per portrait that I need done, so I have a good chance of getting a masterpiece. Sometimes, my sim might have two portraits to do in that time, so I'll have 20 all set up and started. I remember reading about this on the challenge board a while back. I didn't notice a second boost to my painting/s with the opportunity completed twice in a row, nor an additional boost to the painting after that. I suspect that the value from the second opportunity just overrides the first.