Last year, I signed up for the October test, but as the date got closer, I realized I was underprepared, so I signed up for the November test right before the deadline at a location that was about four hours away from me. I was not happy with my October test results, and improved my score fifty points in the November test. The important thing is that the deadline for the November test is BEFORE the October test even occurs. Also, you won't get your October test results until AFTER you take your November test. This caused me a fair amount of grief, though with the whole online reporting of scores now, perhaps they've sped up the process.
Bottom line: if you have a concern about your performance on the October test, sign up NOW for the November one. If you feel like you nailed the October one after you take it, go ahead and cancel the November one. Reporting two test scores won't hurt your chances, though, as admission committees tend to look at the higher score.