What is correct board protocol if a zombie thread becomes highly interesting before being closed? Is it okay to start a duplicate, non-zombie thread with the same subject?
It depends on the forum.
In some of the fora, just continuing a zombie is fine because there is no serious conflict going on.
In a couple of fora, zombies are not favored because there are liable to have been feuds that have been resolved that are re-ignited when a zombie is called up. In those fora, (such as Great Debates), opening an entirely new thread with a link to the original thread is actually the preferred method of handling things.
Nothing on the SDMB suggests that any topic may be discussed only one time. With new posters logging on all the time, there will always be new voices wishing to be heard.
Underlining what tomndebb said, the logic behind starting a new thread is to avoid the situation where someone reads an old thread and responds to someone who posted three years ago, and is no longer reading the boards. Especially in forums like Great Debates and the Pit, this seems unfair to us and feels like potentially picking at very old scabs. And it could be confusing to newer members. If you start a new thread, and comment “There were some ancient comments over here link,” we avoid that problem generally.