Rav said, “Wherever the Torah stated 'law' and 'decree', it means to make the step essential, so that if it is omitted, the whole procedure has to be repeated.” The Talmud asked Rav questions, based on the laws of Nazir, thanksgiving offering, and Yom Kippur, and changed the understanding of what Rav said to: only 'decree' indicates the requirement, but 'law', although similar, does not.
In regard to flour offering, Rav gave an additional rule: any time that the Torah repeated a specific law about flour offerings, it did so to emphasize that it is required. For example, the Torah repeated the requirements of finely ground flour and oil to show that they cannot be omitted. Shmuel disagrees on that. But how could Shmuel disagree with the rule that repetition means a requirement? - Actually, they argue about one point only, the requirement to use one's hand for the handful. It is stated about Aharon and is repeated for the future. Rav says that this is valid repetition, but Shmuel states that we cannot learn new laws from the one-time happening with Aharon.
Art: Emmanuel de Witte - Interior of the Portuguese synagogue