My interest further piqued, I asked my former rabbi George Schlesinger. Here’s his answer.

In 27:11-26, >span class="">which is composed of curses, should not be divided in order to avoid starting or ending on a negative note. Likewise, 28:15-69 should not be divided for similar reasons. Some overlapping is necessary in years II and III and the reading in year III is longer in order to meet the minimum requirement of verses and to avoid unwarranted divisions. Also, in Year II, verses 6-8 in chapter 27 are repeated in aliyah #5 to avoid beginning the aliyah too close to a paragraph ending (i.e. within three verses of the ending). This is necessary in order to guarantee seven aliyot in the Seder where 27:11-25 cannot be interrupted. 

And so not to have you think I’m so smart or “all-knowing”…the answer comes from the original T’shuvah (1995) of Rabbi Richard Eisenberg who “created” the RA’s official triennial reading list.

I know, that really doesn’t make sense.

Thanks for checking!  I’ve been reading Torah for 15 years and never seen overlapping Aliyot. It’s been in the source document all these years, so it must be intentional.