The same person can appear on a chart more than once. A cousin marriage puts them on a descendant chart both as a descendant and as a spouse, and pedigree collapse does the same on an ancestor chart. Normally the only clue is a small bent arrow beside the box, pointing towards the other occurrence.
The Highlight duplicate people option makes the repeats obvious: a soft grey shading appears behind every occurrence of such a person, and a curve links one occurrence to the next.
On the Layout tab of the Chart Properties panel, tick Highlight duplicate people in the Displayed Content section.
The option is off by default and is saved with the chart, so each chart decides for itself.
Each occurrence of a duplicated person is shaded, and a curved line joins the occurrences so you can follow the person from one place on the chart to the other. The links find their own way around other people, and shading and curves alike are drawn behind the chart itself, so nothing they pass becomes harder to read.
The small bent arrows are unchanged — they still point in the direction of the person’s other occurrence.