Kwartierherhaling, literally “repetition of quarters,” means pedigree collapse: the situation where the same people appear in multiple places in your pedigree chart. That means you descend from the same person or couple in different ways.
If your ancestors are from small towns in the Netherlands, sooner or later you’ll run into kwartierherhaling. In general, the more recently your ancestor lived in a small village, the higher the collapse. On my father’s side, who was from the village of Winterswijk, I have about 3,000 boxes in my pedigree chart filled, by only 1,300 unique people. The rest of the boxes are filled in by people who appear in multiple places.
There is one couple, Derck Mierdinck and Armgart Ubbinck, from whom all my paternal great-grandparents descend, in more than a dozen ways between them. Members of this families were serfs originally, who had limits placed upon whom they could married. They could only marry serfs from the same overlord, or pay a fine. That encouraged marrying in a small group, which further contributes to pedigree collapse.
See The Worst Case of Pedigree Collapse Ever? for an extreme example.