Ideally, yes. But I understand this can be particularly challenging, especially when the “someone” is a child.
When a child is involved, you could say something along the lines of, “No thank you, this is mommy’s oatmeal, but I can make you your own bowl of oatmeal if you like.” At other times, you may decide to give them a bite, especially if it’s just going to be one bite and you don’t think you’ll end up feeling deprived afterward (because you’re eating less food). If you can, hop up and get them some of their own.
If it’s an adult who would like to share, oftentimes they’ll say something like, “Would you like to share entrees tonight?” …to which you can simply say, “No thanks.”