This is one of the better-known cases of celebrities looking alike in the world today. Both Rupert Grint and Ed Sheeran are very aware that they look similar, and enjoy playing up to the fact. Sheeran has even had Grint appear in one of his music videos, playing the guitar and generally taking his place. They can't exactly blame their fans for getting things mixed up if they're encouraging it to happen.
If you study them carefully, they don't actually look all that alike in the face; Sheeran's features are softer and rounder, but it's the shock of red hair that makes people look twice. Even though Grint was in one of the most famous movie franchises of all time, he says the ratio of people identifying him correctly is only around 50/50. He must really hate it when Sheeran has a new album to release. Maybe he should just take up playing guitar for real and go with it.