Sometimes, our traditional approach to the reading includes having the congregation recite a short section of the reading aloud before the Torah reader does it, and then he follows. The most frequently occurring version of this is on minor fast days, when the Torah reading includes three such repetitions. We also do this several times on Simchat Torah during the reading of the beginning of Bereishit and also multiple times during the reading of Esther on Purim (which is not a Torah reading per se, but expects largely similar behaviors).
When I am present in the synagogue during the Torah reading, I typically read along quietly with the public reading, as I believe is the optimal practice. [See Update below.] I've always wondered what I'm supposed to do when the Torah reader repeats these sections after the congregation. Should I just listen while he reads because I've already said it, or should I read along with him, because that's what I always do? I'm leaning toward the former because some of these passages break in the middle of a verse, and it seems wrong to stop in the middle of a verse, go back, and start reading from the beginning of another verse.
Similarly, what should I do if I am called up for the Aliya that includes one of these repetitions? Do I recite with the congregation, read with the reader, or both?
Following Alex's answer, I did a little research. It seems from Footnote 154 here that there's actually a disagreement regarding whether a congregant should read along with the reader. I need to take this up with my Rabbi regarding my own practice, but it seems that I have whom to rely on.
And to read "Shnayim Mikra Ve-echad Targum" during the Torah reading is allowed. But all of this doesn't apply to Parashat Zachor and Parashat Para, which are mandated by the Torah, so one needs to focus and hear them from the reader. And the truth is that for all Parshiot, it's proper for one who is careful with one's actions to focus and hear them from the reader.
The Magen Avraham there (#5) quotes the Shelah as saying that reading along is forbidden, and the Mishna Berura (285:14, when the Shulchan Aruch reiterates the same permission) quotes a number of other authorities, including the Gra, who similarly forbid.
On the other hand, he Misha Berura's own opinion is:
However, it seems that to read in a whisper, word by word with the reader, we don't need to be stringent [against], since he will then be focused on hearing every word from the reader. And the Magen Avraham there [on 146] cited the Mateh Moshe saying that lechatchila (in the first instance), it's right to do so.
So it seems that reading along during normal parshiot is either forbidden, permitted, or encouraged, but is forbidden during Zachor and Para. So, I think my question stands with respect to fast day and Simchat Torah reading. It's possible that the answer is "Even if you usually read along, don't read along with those readings, which aren't part of the weekly cycle anyway."
My question also stands regarding Esther, for people who are using their own scrolls.
Also, I believe that my question certainly stands for a person who has an Aliya. See Shulchan Aruch Orach Chaim 141:2:
Two people should not read at once. Rather, either the person who was called up reads, and the reader is quiet; or the reader reads, and the person who was called up does not read aloud, but he does have to read along with the reader so that his blessing is not for naught, so he should read along quietly enough to not be heard by his [own] ears.
The only disagreement apparent on the Mishna Berura page is that the Rama is not even so concerned that the person who was called up needs to be sub-audible. So, it seems that the person who goes up for an Aliya should indeed read along, and my question is unescapable.