There is a prohibition in the Torah not to add or subtract from the commandments of the Torah. There is a four way argument among the Rishonim [mediaeval authorities ] about what this means.  To the Raavad, it refers only to positive commands. The reason is the courts have permission to add restrictions as a fence around Torah.  However as important this subject is, I wanted to add here the fact that Rav Nahman of Breslov mentions a related issue in the Le.M. II chapter 44 and II chapter 86 and in the Conversations of Rav Nahman 235. There it says that one should not seek extra restrictions.   

Most of what the religious world does is adding extra restrictions. And ''כל המוסיף גורע''. ''Anyone that adds [make believe] restrictions ends up subtracting [transgressing real restrictions].'' [That is a common saying in Israel]. in fact even adding restrictions as a ''fence'' is also problematic as is brought in Avot d'Rav Natan on the first Mishna in Pirkei Avot there it says adding a fence around Torah is exactly what Adam, the first man, did. [And that caused the fall of all mankind.]It say there that was wrong, ''Better ten inches that stands, than 100 yards that falls.''

But for many decrees that are from the scribes, the reason for the decree no longer exists, and thus to the Raavad  they are nullified.  [ But to the Rambam, the decree still stands. that comes from beitza pg 5.  but in the gemara in gitin we see that once the reason for a decree i gone, the decree no longer stands.

The Ramban and Raavad, the Rambam and Tosphot are the main sources to look at. but after all is said and done the point that is most important is Rav Nahman's not to keep on adding restrictions all the time like the insane religious world does. 

I would like to suggest that everything about the prohibition of thou shalt not add revolves around the mishna these are the commandment that have no limit. that is there are commandments that once one has done them they are done and one is not allowed to add. there are other commandments like learning Torah that have no upper limit and so the more one does them the better. But for some reason that is not what תוספות says. They  say one can do again even a mitzvah that one has fulfilled. I do not know why תוספות does not make this simple distinction. Especially after the גמרא itself seems to imply this when they ask in סוכה דף כ''ח why is it that if one sleeps in a סוכה on the eighth day of שמיני עצרת he does not get lashes?

 

 אני רוצה להציע שהכל על איסור "אל תוסיף" סובב סביב המשנה שאלו המצוות שאין להן גבול. יש מצוות שברגע שעשה אותן הן נעשו ואסור להוסיף. יש עוד מצוות כמו לימוד תורה שאין להן גבול עליון ולכן ככל שעושים אותן יותר טוב

 i mean  תוספות ראש השנה ט''ז ע''ב says one can do a commandment more than once, but i think this must refer to commandments that have no upper bound. 

כלומר תוספות ראש השנה ט''ז ע''ב אומר שאפשר לעשות מצוות יותר מפעם אחת, אבל אני חושב שזה חייב להתייחס למצוות שאין להן גבול עליון

אבל משום מה זה לא מה שהתוספות אומרים. הם אומרים שאפשר לעשות שוב אפילו מצווה שקיימה. אני לא יודע למה תוספות לא עושות את ההבחנה הפשוטה הזו. בפרט לאחר שהגמרא עצמו נראה רומז לכך כששואלים בסוכה דף כ''ח למה זה שאם ישן בסוכה ביום של שמיני אצרת אינו מקבל מלקות?