Why the Repetition?
The author of the Haggadah that we recite on Passover night asks rhetorically:
“The paschal lamb that our ancestors ate when the Holy Temple stood – is for what reason?
Because G‑d passed over our fathers’ houses in Egypt, as the verse states, “You should say ‘It is a Pesach-offering to G‑d, because He passed over the houses of the children of Israel in Egypt when he struck the Egyptians, and he spared our households.’ Then the people bowed down and prostrated themselves” (Exodus 12:27).
Commentators ask, why does the Biblical verse have to add the words “and he spared our households?” Isn’t it obvious that if G‑d passed over the houses of the children of Israel that their houses would be spared?
Moreover, the Haggadah itself only mentions that G‑d passed over the houses of our fathers. Why did the author of the Haggadah not follow the Torah’s repetition of the statement that He passed over the houses of the children of Israel?
Two Plagues
Beis Halevi answers the first question based on his observation that the many corpses created a health threat which took many lives.
Thus, the Torah states that in addition to the fact that the houses of the children of Israel were spared the plague of the first born, their houses were also spared the ensuing plague that spread to the general population.
This answer however, does not address our second question, why does the author of the Haggadah fail to state that no one suffered collateral damage from the secondary plague, as the Torah seems to suggest?
Diverse Time Periods
One may suggest a deeper explanation that takes into consideration the different time periods involved here:  
The Torah’s explanation comes in response to the question of children who ask, “What is this ritual service to you?” The question is posed by the child identified as the Rasha, the rebellious son who cannot relate to the Seder, who is challenging his father, one of the children of Israel who lived in Egypt at the time of the Exodus. Conversely, the child to whom the author of the Haggadah speaks is one who grew up in the Holy Temple-Bais Hamikdash era.
The State of Nakedness
When the Jews were enslaved in Egypt they suffered under Egypt’s impure and stifling influence. Indeed, the very reason they had to bring the Paschal lamb, our Sages tell us, is that when the Jews were to be released from bondage they were spiritually “naked.” Our Sages base this assessment of the pitiful spiritual state of the children of Israel on the words of the prophet Ezekiel (16:7), cited in the Haggadah, which described the children of Israel as “naked and bare.” They therefore, needed a Mitzvah to bring them enough worthiness to warrant their redemption. G‑d granted them the Mitzvah of the Paschal offering, together with circumcision, to raise their spiritual state.
The Jewish people were so immersed in idolatry that the Divine attribute of judgment complained to G‑d.  Why was He sparing the children of Israel even as He was destroyed the idolatrous Egyptians? The angels too argued that “These and those are idol worshippers!”
Yet, G‑d overruled His own attribute of judgment, which is precisely what the word Pesach-Passover means. It does not just suggest that G‑d skipped over the houses of the children of Israel. It also means that He passed over, overrode and overruled His own system of justice, logic and fairness, as it were, to spare the children of Israel.
Getting the Egypt Out of the Jew
Now, despite the fact that the children of Israel brought the Paschal offering and left Egypt, they still needed G‑d’s help to fully expunge the insidious influence of the debased, pagan Egyptian culture.
The Rebbe has noted that the existence of a Rasha after the Exodus was due to the fact that when they left the land of both physical and spiritual affliction, the children of Israel took some of these afflictions with them. This, the Rebbe explains, is the deeper meaning of the opening paragraph of the Haggadah, “This is the bread, [which is emblematic of their] affliction, that our ancestors ate in the land of Egypt,” and is the same state in which they left Egypt, which explains why we still have physical and spiritual poverty and bondage. To paraphrase a famous line: “while you can get a Jew out of Egypt, you can’t [easily] get Egypt out of the Jew.”
It was this residual malign influence of Egypt that caused some Jews to degenerate into open rebellion against celebrating the Exodus.
Two Responses
Thus, the Torah tells the father to tell the rebellious child that G‑d did two things: first, He did not subject us to the same fate as the Egyptians whose flaws we shared. This relates to the physical salvation.
Moreover, He also saved our homes. The Torah does not say “He spared our lives” for that meaning was already conveyed by saying that He passed over the homes of the children of Israel when struck the Egyptians. Here the intent is that he spared our homes, which doesn’t just mean that He did not destroy our homes, for He did not destroy the homes of the Egyptian first-borns either. Rather, the meaning of house here is symbolic. It refers to the spiritual shelter that protected the children of Israel from losing their identities, and which allowed them all, even the rebellious ones, to be lifted out of the Egyptian morass.   
Moreover, the Hebrew word for “spared” (hitzil) also means “separated.” G‑d didn’t just spare us, He separated us from Egypt to be part of the Chosen People and to bring G‑dliness to the world.
We must tell the exile Jew, lovingly but firmly, that he or she is part of our Jewish Home and doesn’t belong any other place. Ultimately, even the Rasha will realize that he is a Jew who belongs in the Divine House, and he too will be liberated from exile. The message of Passover is that G‑d spared you, separated you and placed you in His Home.
In the Palace
By contrast, in the Bais Hamikdash, which G‑d’s presence permeated, there was no need to tell the child that he or she was part of the Jewish home. That was a given; the Bais Hamikdash is the starkest reminder of that, for it is the ultimate Divine structure. Only a homeless person must be told that he needs to live in, and indeed has, a home provided for him. A person who lives in a palace obviously does not have to be told that his place is in a home.
The Bais Hamikdash-era child wants to know the deeper significance of the Paschal offering and need not be told that he is different.  
Our response to the child of the Bais Hamikdash is that G‑d’s relationship with us transcends the boundaries of the natural order. The “Houses of our fathers in Egypt,” in this context, can be understood metaphorically as referring to the structures of nature and logic. Partaking of the Paschal offering would “plug him in” to that transcendent Divine energy.
This year, let us hope and pray that we will be blessed to eat the Paschal offering in the Third Bais Hamikdash with Moshiach at our head.