Having mentioned the Mitzvah of keeping Shabbos at the beginning of the Parshah, why does the Pasuk repeat it here?
Rashi (from Yevamos, 6a): To teach us that in spite of the Kavod of the Beis-ha'Mikdash (which the Pasuk is about to mention), one may not construct it on Shabbos. 1
Ramban, Moshav Zekenim: The Torah warns against Chilul Shabbos many times, because, like Avodah Zarah, it is equivalent to all the Mitzvos combined - and anybody who does not keep Shabbos it is as if he denies the fact that Hashem created the world, and has no Torah at all.
Seforno (on Pasuk 29): After warning against desecrating Yisrael's sanctity, the Torah issues commands to honor the days and the locations that are holy. 2
Ibn Ezra, Moshav Zekenim: It is put near 3 Misah (verse 28) to teach that mourning does not apply on Shabbos. Likewise [fear of] the Mikdash was taught here to teach that the Kohen Gadol and people of the Mikdash not to conduct like other mourners.
Moshav Zekenim: It is put after "u'Mal'ah ha'Aretz Zimah", because the festivals are the worst times for Zimah (Kidushin 81a), and also Shabbos, when people eat and drink - "Zenus v'Yayin v'Sirosh Tikach Lev" (Hoshe'a 4:11). It is put before "Al Tifnu El ha'Ovos" because they do not answer on Shabbos.
See Sifsei Chachamim.
Seforno: Hence "My Shabbasos" incorporates the Yamim-Tovim, and "My Mikdash", all holy places - such as Batei Midrash and Batei K'nesses - thereby accentuating the sanctity of time and space.
Ibn Ezra explains that also "Al Techalel Es Bitcha" (verse 29) needed to be taught near Seret la'Nefesh, for wounding one's daughter over a Mes will lead to Zenus, but others rejected this (Karnei Ohr).
What are the connotations of "Mikdashi Tira'u"?
Rashi: It is a Mitzvah to treat the Beis-ha'Mikdash with awe, by not entering it with one's staff, wearing shoes or a money-belt, or without first washing one's feet.