Øystein Morten, a norwegian historian that wrote a book about Magnus I Olavsson «the Good» den Gode say that they are two different women.
Page 28 and on: Ælfgifu of Northampton, she was the first wife of Knut den mektige. Her father was Earl Ælfheim, ruler of Northumbria and he had been one of the closest advisors to the english king Ethelred. Ælfgifu was partly raised at the english court.
Knut married Ælfgifu in 1013 to get an alliance with the most powerful family in North-England. Their son Svein was born the year after. Ælfgifu looked forward to be Englands next queen, and Svein was the foremost heir. But, after Knut had conquered the whole of England in 1016, he married the widow of King Ethelred - Emma, the daughter of the Duke of Normandy. Ælfgifu was set aside. Knut didn't divorce her, she still was his wife, but she became second in rank, and Svein was ranked below the children Knut had with Emma. The church started to complain about Knut having two wives, and he sent Ælfgifu and Svein abroad. They were among other things sent to rule Norway in Knut's name in 1030-1035 when they fled from Magnus. They fled to Denmark where the halfbrother of Svein, Hardeknut, rules, he is 17 years old, while Svein is 21. Hardeknut is the oldest son of Knut den mektige and Queen Emma of Normandy. Hardeknut doesn't want to help his halfbrother against Magnus, mainly because he is afraid of Magnus having a lot of supporters from todays Sweden and Russia that can attack him in Denmark, and he wants to consolidate his power in Denmark. Ælfgifu and Svein are stowed away on a small farm, they lose their forces and ships and here they hit rock bottom. After a short period, Svein gets ill and dies, and a short time thereafter Knut den mektige dies in England. Ælfgifu now see a chance and travels back to England and helps her son Harald Harefot to power, since he is the only son of Knut present at his funeral.