Ulcers occur when stomach acid damages the lining of the digestive tract. It is similar to the reaction on our skin when exposed to acid. So, you should stop the practice of agnisara. Agni sara, being abdominal movements will not cause any benefit to the lining of the intestine, rather it can cause damage to your duodenal ulcer. Agni sara kriya increases the body heat excessively. So, it is better to avoid this practice. Though to counter the heating effect and raised blood pressure, it is usually suggested to follow up with cooling pranayama - like sheetkari or sheetali pranayama. But I could not suggest any pranayama which could help you specifically as I do not know about the stage /time of duodenal ulcer alongwith medical case history. Nor you should perform any asanas like chakrasana, ushtrasana, which have stretching effects on the intestines.
I hope this helps. However, you must consult your physician for ulcer and exercises.
