Other people are external objects, just like all other physical objects are. A skeptic does not grant you knowledge of other people and then say 'we' can't know of the existence of external objects. Sense data exist in the individual mind. No two people can have the same sense datum. Sense data exist only when being perceived.
Sense data vary depending on observer's condition.
The objects that you see change depending on your perspective. Physical objects do not change depending on your perspective. Therefore, the objects that you see are not identical to the physical objects. When you have a case of double vision, you see two of something. Therefore, what you see is something non-physical.