I got this Annotated Screen play book for all three of the original movies and In ROTJ originally in the open crawl two Death Stars are menchioned in the rough draft, but it was changed to one. Lucas known for recycling his ideas could possibly in Episode III have a reference made that they plan on building many Death Stars,That way when we see ROTJ we wont think how did they build that so fast if the first one took 20 years to build......(hope that made since iam dog tired)