Blood pooling. You probably have some degree of blood pooling in the lower extremities which increases with the inactivity of prolonged sitting. When you stand up suddenly your upper body, and this includes the eye muscle coordination center in the brain stem, has a temporary loss of blood and this alters the function which for you causes a brief doubling of vision. Try getting up slowly/flex your muscles first.
Maybe. Anything sustained is not normal. So if it's just a few second of double that clears right away, then it's probably not serious and could be related to blood flow. Standing up quickly after working long hours--it sometimes takes a few seconds for the blood flow to catch up. Of course, it's best to get it checked out if your vision is abnormal.