Using something like HaShovas Aveida as the example, why does the Torah give both positive AND negative commandments to the same mitzvah? I.e. Why tell us that we must return a lost object and then tell us that we are forbidden from choosing NOT to return it? Wouldn’t the commandment to return it in and of itself teach us that we are forbidden from choosing not to return it?
There are a number of mitzvos in the Torah for which there is both a positive and negative commandment, such as tzedakah (the obligation to give a loan to poor people), returning lost property, Shabbos, and others.
This gives the mitzvah added severity. We are commanded not only to “do good,” but also to “refrain from bad.”
Moreover, these are two different dimensions of the mitzvah, and neither is extraneous – on Shabbos we are obligated to rest on one hand, and to avoid labor on the other. These are not identical, and they carry two different messages and imperatives. The same is true for the positive mitzvah to return lost property, and the prohibition against ignoring another’s loss.
In addition, there are important ramifications of the combined positive-negative commandments, in particular where there is a clash between the mitzvah and other mitzvos.