We've already got alot over ovelap. Looking only at the Roman Empire, substantially overlapping civs:
I) Greece, Egypt, Carthage, Celts, Spain, France, England and Ottomans. The core areas of all these belonged to the Roman Empire.
II) Germany, Persia, Arabia. Large bits of modern Germany and today's Arab world belonged to Rome. The eastern half of what became the Roman Empire was under Persian control under the Achaemenids.
Smaller overlaps exists with Babylonia (Mesopotamia under Roman control under some periods, and Syria-Palestine under Babylonian for a while) and Russia (the Crimea, and bits of the Caucasus area).
Conquests will add Portugal, Netherlands, Hettites (which will belong to category I above), Sumeria (small overlap), and Byzantium (successor state).