Why did Moshe need to teach here that a separate dipping was required for every application of the blood?
(12,22) “And you shall take a bunch of hyssop and dip it in the blood which is in the סף (basin), and you shall touch the lintel and the two doorposts from the blood that is in the סף (basin).”
Rashi brings the Mechilta which asks why the phrase “from the blood that is in the basin” is repeated, and answers that otherwise you might think that the Torah requires only one dipping for all three applications. Hence it repeats the phrase “from the blood which is in the basin” to teach you that for every application of the blood a separate dipping is required.
But why is this teaching not alluded to earlier in posuk 7, when Hashem instructed “and they shall take from the blood and put it on the two doorposts and on the lintel”? Why did the Torah wait until this posuk to teach it?
But it seems to me that according to the opinion in the Mechilta that the word סף means threshold (meaning that a hole was carved out in the threshold of the house and the blood was poured into it) we can explain that there was no need to teach this in the earlier posuk, but in this posuk it needed to be taught. Because the difference between our posuk and the earlier posuk is well known - there it says “and place it on the two doorposts and on the lintel”, but here it says “on the lintel and on the two doorposts” - Rashi explained that the order was changed in order to teach that if the application of the blood was not done in the correct order, the application is still valid.
Now, there is a rule that one should not pass by a mitzvah. For this reason the blood which remained from the sin offering that is offered within the Heichal (the inner temple) was poured on the western base of the altar, since this is the part of the altar that the Kohen Gadol first meets with when he emerges from the Heichal. There is also a rule (with regard to mitzvos) that one should always turn to the right .
Therefore, from Hashem's instruction in the earlier posuk that the blood should be placed “on the two doorposts and on the lintel” we could have known from logic that every application of blood required a separate dipping. Because since the blood was placed in the threshold of the house, logically the blood was to be appled first to the doorpost since that is the first thing that is met with. Not only that, but it was to be the righthand doorpost according to the rule to always turn to the right.
But this creates a problem, because according to this same rule he should continue to the right and make the next application of blood on the lintel, and finally on the lefthand doorpost. So why does the posuk say to place the blood first “on the two doorposts” and only afterwards “on the lintel”? Perforce, the explanation must be that after the blood is placed on the righthand doorpost a new dipping in the blood is required, and thus the next application is to be placed on the lefthand doorpost which is closer to the threshold than the lintel. And it follows from this that another dipping in the blood was required before the application on the lintel.
Hence, logic would have taught us that every application of the blood required a separate dipping, and no separate teaching would have been needed to teach this. But when Moshe changed the order in our posuk and instructed Yisrael to place the blood “on the lintel and on the two doorposts” they would not have been able to learn from logic that each application required a separate dipping. Thus, the Torah had to teach this explicitly.