Or HaChaim on Numbers 17:15/אור החיים על במדבר י״ז:ט״ו

1 וישב אהרן…והמגפה נעצרה. Aaron returned and the plague had been arrested. The reason the Torah repeats once more that the plague had been arrested is to inform us that even after the cloud of smoke from the incense had dissipated the plague did not break out again. We should not think that the effect of Aaron's offering was limited to while he was in the process of offering incense. We might have thought so in view of what the Torah told us in Exodus 17,11 that Israel's army prevailed over Amalek only while Moses managed to keep his hands raised heavenwards.

2We may also understand this repetition in light of the Midrash Tanchuma on that verse who claims that the angel of death refused to take orders from Aaron until Aaron brought him to the entrance of the Tabernacle where he heard from the voice of G'd that he was to desist. The meaning of the word והמגפה would then be a reference to the angel of death who is defined as "the plague." At this juncture the angel of death agreed to become inactive, i.e. to stop killing the people of Israel. Whereas in the previous verse the Torah wrote ותעצר המגפה, in this instance the Torah wrote והמגפה נעצרה. In the previous verse the Torah refers to Aaron's role in stopping the plague although the angel of death had not agreed yet; in this verse the Torah describes that the cause of the plague, i.e. the angel of death himself agreed to desist.