For most beginning toastmasters the great goal set before them is completing their CTM. After the CTM there are an array of possible goals: any of the 15 advanced manuals, the ATM program, the CL award for leadership, district speech contests, etc. With all these possible new goals to work on why would someone want to repeat the already completed achievement of the CTM?
In my case it was because having completed the CTM I looked back and realized that I could do SO much better a job on those early speeches now. The lessons I'd learned from the speeches for my CTM made me better able to learn from doing the CTM speeches.
Can a person repeat the CTM manual? Certainly! Not only can you repeat the CTM manual as many times as you like, but you can receive credit for a CTM each time you do so. Not only are there people who have done the CTM two or three times there are people who have done it dozens of times. The record for achieving CTMs in district was set by someone doing over 50 (yes FIFTY) CTMs (regrettably this member has since retired to Florida but this is still a wonderful goal to shoot for).
I am now working on my third CTM because I have again looked back and decided that the basic skills taught by the speeches for a CTM are so fundamental, and so educational, that a review is now beneficial to me. Each time a member completes one of the speeches from the basic manual towards a CTM, whether the first CTM or the fiftieth, valuable skills are reinforced. The speeches in the basic manual are the fundamentals of all effective public speaking. To ignore those basics is to ignore all public speaking skills.