Uniqueness

My two favorite kinds of science fiction are time travel and future history series: Wells and Heinlein.

Time Travel
There are two fundamental premises: the past either can or cannot be changed. Poul Anderson systematically examines both. In The Corridors Of Time and There Will Be Time, rival groups wage war throughout an immutable timeline by changing the significance of known events whereas, in the Time Patrol series, an organization prevents change in a mutable timeline.

Future History Series
Robert Heinlein wrote one Future History, although five early Scribner Juveniles share a background with each other and with the "Green Hills of Earth" period of the Future History. Thus, these five novels might count as a "Juvenile Future History."

James Blish wrote a four novel future history, Cities In Flight, a four story future history, The Seedling Stars, and the non-linear Haertel Scholium containing three distinct future historical sequences.

Larry Niven has the Known Space and the State/Smoke Ring histories.

I think that other future historians have one series each except Anderson who has at least eight. I am here counting the single novel, Genesis, as a series because its successive chapters cover geological ages. Anderson moves away from histories with FTL, aliens and interstellar empires towards histories with STL, no aliens and human/AI interactions.

These thorough treatments of time travel and future histories make Anderson unique among sf writers.