2,648. An Oath on Top of an Existing Oath
Hilchos Shevuos 4:10
If someone takes an oath not to eat on a given day, and then he takes an oath not to finish a certain loaf, even if he eats the whole loaf on that day, he is only liable once; the same is true in all comparable cases because an oath doesn’t take effect on top of an existing oath. However, if he took an oath not to finish a loaf, after which he took an oath not to eat anything or not to eat a particular loaf, then he is liable twice. This is because when he took the oath not to finish a loaf, he would only be liable for eating the whole thing. Accordingly, when he took an additional oath not to eat anything or not eat that particular loaf, he is liable for eating the volume of an olive. When he finishes the loaf, he is then liable for his original oath.
Hilchos Shevuos 4:11
Let’s say that someone took an oath not to eat figs, after which he took an oath not to eat figs and grapes. In such a case one would be liable twice for eating figs because he included figs – which were prohibited by his first oath – with grapes, which were permitted to him. Since his second oath was effective vis-à-vis grapes, it is also effective vis-à-vis figs and he is liable twice. Imposing liability on top of liability was previously addressed in Maachalos Assuros 8:6.