Under the naive assumption that you have two parents, and each of them had two parents, who were born to two different sets of parents .. well, in a few generations the expected population of your family tree will become unexpectedly high: perhaps, more than the number of humans who have ever lived on the Earth.
But clearly, that isn't the case. So what's happening?
This is explained by "Pedigree Collapse", a concept coined by Robert C. Gunderson. According to Wikipedia,
This apparent paradox is explained by shared ancestors, referred to as pedigree collapse. Instead of consisting of all different individuals, a tree may have multiple places occupied by a single individual. This typically happens when the parents of an ancestor are related to each other (sometimes unbeknownst to themselves).[3][4] For example, the offspring of two first cousins has at most only six great-grandparents instead of the normal eight. This reduction in the number of ancestors is pedigree collapse. It collapses the ancestor tree into a directed acyclic graph.