According to the view (quoted here) that the reading of the Targum in shnaim mikro v’echod targum is to
add layers of explanation to every word
could one fulfil the mitzvo by reading the Targum of the offerings of the Princes in Parshas Naso only once? One would read the verses that contained the name of the Prince but then skip the rest of the Targum until the name of the next Prince.
See Mishne Berurah 285:3 that even names like Reuvain and Shimon are read שנים מקרא ואחד תרגום. This is based on the Gemara Berachos 8 that even "Ataros veDivon" (city names) whose Targum does not add anything are included in the mitzva of שנים מקרא ואחד תרגום.
The parshios hanesiim clearly have the same halacha and are read in Targum.
On a logical basis, the repetition of the pesukim is in and of itself necessary and teaches a lesson. Thus, it would appear that in order to fulfill שנים מקרא אחד תרגום one would have to do the targum for each pasuk as one does the mikre. If one were to be able to skip the targum then one would be able to skip the mikre. Another example could be Vayedaber Hashem el Moshe Laimor. You could ask the same question about that.
The Gemara (Brachos 6a-6b) explicitly obligates shnayim mikra on proper nouns.
אמי לעולם ישלים אדם פרשיותיו עם הצבור שנים מקרא ואחד תרגום ואפילו {במדבר לב-ג} עטרות ודיבון
The reason for this law is unclear. Out texts of the Gemara and the Rif have the next line in the Gemara providing rationale: for anyone who completes his portions with the community will have his life extended.
שכל המשלים פרשיותיו עם הצבור מאריכין לו ימיו ושנותיו
However, this seems like backwards reasoning. The reward follows the enactment not the other way around! (i.e. one gets reward because that was the institution of shnayim mikra, the institution of shnayim mikra doesn't depend on the reward).
The Seder Rav Amram Gaon (Harpenes; Shacharis of Shabbos) has an alternative text in which the second line is its own statement. Accordingly the question of why we must repeat proper nouns remains unanswered. (See ROSH for a similar text).
