It's a simple matter of math, geography, and settlement patterns:

The numbers are pretty easy.

There is 1 of you--unless you've got an identical twin

Two biological parents

Four biological grandparents

Eight biological great-grandparents

The number keeps doubling each generation. Counting yourself as

generation one, by the time you have reached generation twenty, the

seventeenth great-grandparent generation has been reached. There are

524,288 theoretical blank spots in this generation of your pedigree

chart. Depending upon how "mixed" your ancestry is, there's a

reasonable chance of repetition. Extend the lineage back ten more

generations and the number is a staggering 536,870,912 ancestors! Now

there's a database (documenting it is another story). Thirty generations

ago, the population was significantly less than half a billion. If you could

trace each line that far (and chances are you can't), there would be

names repeated.

When one keeps in mind the small

geographic area these individuals came from the number of "repeat"

ancestors is not surprising. When records allow tracing ancestors for

two or three hundred years in a village of two or three hundred people,

the chance of intermarriage is great. If individuals from a small village

migrate to the United States together (as some individuals from this

area did) the geographic closeness may be replicated (at least for the

first few generations).

Even if you are not related to yourself, it's possible that you are related

to an individual in more than one way. There are many individuals who

are "double first cousins" (where brothers married sisters, for example).

The relationship may get even more complicated than that. 

Should we be telling people about all these double cousins??!! HA!