Sadly people having a do-over and having a Big White Wedding after a small, justice of the peace one seems too common. My husband had a close friend who did this. They got married down at city hall and then later regretted not having had a big deal, so they threw a big, expensive wedding over a year later.
Of course people are free to do this; the only thing I find off about it is that it does seem very attention-grabbing, gift-grubbing usually. If you already gave a gift, there's no reason to give another. Attend if you can, send a note telling them how happy you are for them.