"Incident" is a pantoum, a poetic form that's built on repetition. In a conventional pantoum, the second and fourth lines of the first quatrain recur as the first and third lines of the second quatrain; the second and fourth lines of the second quatrain recur as the first and third lines of the third quatrain; and so on. Also, the first and third lines of the opening quatrain often come back as the fourth and second lines of the last quatrain, as they do here. (For more details, see the Form section of this guide.)
In "Incident," as in many pantoums, the lines don't always repeat word for word; some contain variations the second time around. Here, the variations relate to the poem's thematic focus on memory and storytelling. That is, the way the lines alter slightly as they repeat mirrors the way memories and stories tend to alter slightly with each repetition.
Sometimes the changes also illustrate time shifts in the narrative, as when "the charred grass now green" (line 4) becomes "the charred grass still green" (line 7), signaling a flashback to the moments before the cross-burning scorched the grass.
Even when words repeat exactly, their meaning can subtly differ. In line 16, for example, "the flames had [...] dimmed" refers only to the flames in the hurricane lamps, whereas in line 19, "the flames had dimmed" could refer both to the flames in the lamps and the flames on the cross. This subtle shift is an example of the careful, indirect language through which the poem narrates a traumatic event.
Finally, some lines repeat verbatim. For example, the first and last lines—"We tell the story every year"—are the same, suggesting that some things (like the emotional significance of the memory) don't change.