It's clear that the main requirement on Shabbat, Mincha, (same for Mon & Thurs morning) is
A. To read a minimum of 10 verses(except 9 when it closes the topic, such as with the reading about Amalek on Purim which starts and ends in 9); and
B. To have only 3 people read, not 4. (Which is done on Rosh Chodesh.)
If an error occurs as stated, we do not re-read the mistaken additional verses because that would indicate that the first reader was, for some reason, improper. His reading did have (a) at least 3 verses and (b) blessings were properly said before and after. So... The appropriate thing to do would be to simply continue on past
"Sheini," at least 3 verses, until a natural stop or pause in the topic.
This is done for many parshiot, such as Va'etchanan, Nitzavim and Va'yeilech.
[note: if however the reader read only two verses then his closing blessing is inappropriate. He should read a third verse and then repeat the closing blessing.
If a second reader was already called and said the first blessing before reading, then the first two verses read by the earlier reader "were not blessed properly;" therefore, they must be repeated, as if they had not been read at all! The second reader re-reads the two verses that had been read incorrectly, adds one more for the first reader, then adds 3 or 4 (or more) for himself and leaves the remainder for the third reader,
Source: Shulchan Aruch, Orach Chaim, סימן קלז - כמה פסוקים צריכים לקרא לכל אחד