Why are the dinim of the mikdash recorded twice in the Torah? (Shemos, 25-31 & 35-40).
According to Ramban, ma'amad har Sinai had two functions: (1) the conversion of B’nai Yisrael and (2) the establishment of a covenant between God and B’nai Yisrael such that God would dwell in the mishkan. (Ramban, Shemos 35:1 and elsewhere).
God established the covenant by instructing B’nai Yisrael in the laws of the mishkan (ma’aseh krisus bris). Because God’s residence in B’nai Yisrael is predicated and dependent on their obedience to His commandments, any covenant with B’nai Yisrael must be initiated through Talmud Torah, by reading a sefer ha-Bris. (Shemos 24:7; see also Rav Schachter’s two shiurim on purim from this year). Similarly, God crafted the luchos, the physical embodiment of the covenant (cheftza shel bris). Whereas the conversion of B’nai Yisrael took effect immediately and permanently, the covenant between God and B’nai Yisrael did not. The covenant could only go into effect when the mishkan was completed. Until then, however, it was in limbo.
Therefore, by worshipping the egel ha-zahav, B’nai Yisrael nullified the covenant. God no longer had an obligation or even a desire to dwell among them. (Ramban, Vaiykra 25:1). Because the luchos represented the covenant between God and B’nai Yisrael, when Moshe saw B’nai Yisrael worshipping the egel ha-zahav, he broke them, knowing the covenant no longer held.
After much prayer, Moshe had secured a renewal of the covenant: God agreed once more to dwell among B’nai Yisrael in the mishkan. The covenant, however, likewise needed to be initiated once more. Although B’nai Yisrael were already familiar with the dinim of the mishkan, God repeated them a second time as an initiation into the new covenant — the ma’aseh krisus bris — and provided a second set of luchos — the cheftza shel bris. The completion of the mishkan constituted a hashlamas bris, after which, God was then obligated, so to speak, to reside in the mishkan. Thus, although B’nai Yisrael would commit idolatry in the future, the covenant was sealed and forever binding.