I thought it was ok to do every random once in awhile, like if you had plans that were on shakey ground. Like one time we had a big family gathering (12 people), and we were afraid some members of the family would crash the plans, so we made a backup just for the two of us. ALTHOUGH, the moment I knew one wasn’t going to be used I called and cancelled. Good thing we had the back-up b/c naturally his family decided they didn’t want to go, so I had to call and cancel the 12 people one. Daniel and I dined alone, while they all had to eat hot dogs somewhere b/c they were too indecisive.
I would never do it for the reason of “hmm, not sure what we’ll feel like on this day so let’s make 2!” That’s just kinda inconsiderate in my opinion.
Otherwise, I’d only do it if I knew there could be party complications & cancel the other one right away.