Polyps in the stomach are not very common as compared to the colon, but do occur. Most of them are non malignant and are called adenomas, and the diagnosis is made by endoscopic biopsies and can be completely removed by snaring through the endoscope. Once noted it is very important to have them removed, as they are a very potential source of bleeding which if continues in small quantities for a time can cause anaemia in varying degrees. Rarely the solitary benign polyps can become cancerous and the polyp needs to be removed when the diagnosis is made. I have not discussed malignant stomach cancers here, as they are different and do not present as solitary polyps.
Benign non-malignant duodenal polyps are very rare and need removal and your mothers anaemia could well be due to bleeding.
Finally, it is important for your mother to have full gastro-intestinal tract checked to ensure that she has no more polyps in the small bowel or colon as the duodenal polyp could be part of a very rare condition called multiple polyposis . I suggest you consult a gastroenterologist for appropriate screening.