Are you required to serve a second term on a jury if you have served in the past?

Yep. You just one a lottery. Purely by chance did you get picked.

You can be exempt from jury duty if you have already served on a jury within a certain time, usually 3 to 5 years. The law varies greatly in different states or provinces so check the laws in your area on the internet if the exemptions are not listed on your summons.