If you draw the usual family tree, you can potentially go back too far, where there are more people in your family tree than there were people on the planet at the time. Yeah, this family trees can't grow exponentially forever, and yes, some of our ancestor were related to each other. But that's no reason to panic.
The YouTube channel It's OK to Be Smart explain how human ancestry and DNA diversity really works. -via Geeks Are Sexy
At this point it's kind of outdated, because it was written before we knew the extent of interbreeding between early human subspecies (Homo sapiens, neanderthals, and denisovans), but it's still worth the read. It will answer your questions.
So, Alex, Plasmagryphyn, what do you think about Mitochondrial Eve? Would you consider her the first human or perhaps the last pre-human?