The Mordechi (Rav Mordechai Ben Hillel, Germany, 1250-1298), in Masechet Megila, tells that there was a synagogue in Europe that on Simhat Torah would repeat a section of the Torah reading many times over, and Rabbenu Efrayim (Regensburg, Germany, 1110-1175) strongly condemned this practice and angrily left the synagogue. However, Rav Hai Gaon (939-1038) proved from several passages in the Talmud that it is permissible, when necessary, to repeat segments of the Torah reading. For example, on Rosh Hodesh we repeat the verse "Ve’amarta Lahem," and on Hol Ha’moed Sukkot and Hannukah we repeat text during the Torah reading. According to Rav Hai Gaon, then, it is entirely permissible to repeat text of the Torah reading for the purpose of adding Aliyot, and Berachot may be recited over these Aliyot. Of course, even according to this view, repeating text cannot count towards the seven required Aliyot on Shabbat. However, if it is necessary to add to the seven Aliyot, Rav Hai Gaon permits repeating text, and this is the position accepted by the Shulhan Aruch (Orah Haim 282:2). The Rama (Rav Moshe Isserles of Cracow, 1525-1572), by contrast, writes that the custom among Ashkenazim is not to repeat text to add Aliyot.
Notwithstanding the lenient ruling of the Shulhan Aruch, Hacham Ovadia Yosef writes (Hazon Ovadia, p. 230; listen to audio recording for precise citation) that it is preferable to avoid this debate by breaking the Torah portion into smaller segments. If a Ba’al Koreh (reader) knows ahead of time that additional Aliyot are needed, he should make more frequent breaks, after small segments of text – following Halachic guidelines in this regard – in order to increase the number of Aliyot. This is preferable to repeating text, the propriety of which, as mentioned, is subject to a debate among the Halachic authorities.
Hacham Ovadia adds that although there is a famous principle of "Safek Berachot Le’hakel," which means that we do not recite a Beracha if there is some uncertainty as to whether it is warranted, this rule does not apply in cases where there is an established Minhag (custom). Since there is an accepted Minhag to repeat text to add Aliyot and recite the Berachot, we may follow this practice without concern. Nevertheless, it is preferable to avoid this practice when possible, as discussed.
Summary: If it is necessary to add Aliyot in the Shabbat Torah reading, it is permissible to repeat portions of the reading numerous times, but it is preferable to break up the text into smaller segments, instead.