Why do we have to read weekly Parsha twice in Hebrew and once in Aramaic?

Shalom, Thank you for your question. The source for the mitzvah of reviewing the weekly Torah reading by reciting it twice in Hebrew and once in Aramaic (Shnieim b'mikrah v'echad targum) is found in the Talmud, (Brachot 8) which I quote to you here (in an extended translation that includes explanatory sentences interlaced into it, taken from the Koren English Talmud):- "Rav Huna bar Yehuda said that Rabbi Ami said: A person should always complete his Torah portions with the congregation. The congregation reads a particular Torah portion every Shabbat, and during the week prior to each Shabbat, one is required to read the Bible text of the weekly portion twice and the translation once. This applies to every verse, even a verse like: “Atarot and Divon and Yazer and Nimra and Ḥeshbon and Elaleh and Sevam and Nevo and Beon” (Numbers 32:3). While that verse is comprised entirely of names of places that are identical in Hebrew and Aramaic, one is nevertheless required to read the verse twice and its translation once, as one who completes his Torah portions with the congregation is rewarded that his days and years are extended. … Rabbi Yehoshua ben Levi told his sons: Complete your portions with the congregation, the Bible text twice and the translation once." This law is also found in the Shulchan Aruch (Orech Haim, 285). There are various reasons given for this practice, but the simplest is that by doing so a person connects themselves to the Torah through a collective study of the entire Jewish people. This double connection (to the Torah and to the Jewish people studying together) strengthens our understanding that the two are inseparable. As for reading the Aramaic translation, many are of the opinion that one can fulfill this obliagtion by reading a commentary to the Torah, ideally Rashi. None the less, reading the Aramaic is praiseworthy. For those who would like to ease themselves into this mitzvah, may I suggest the following – one should start by reading one of the seven "aliyot" (sections) of the weekly Parsha each day. On Sunday read the first, and then on Monday the second etc. On Friday one can read the last two together, or read the last one on Friday night or Shabbat morning before shule. In this way one will come into Shabbat having read the Parsha. According to one's level, you could read the section in Hebrew, English and with a good commentary (there are so many out there). I cannot stress how much this will add to your Shabbat. Blessings.