As you may know, some women claim that they have their period while they are pregnant. In the arab world they call it "Deer pregnancy", since when the deer gets pregnant, it still gets its' monthly cycle.
Their doctors do confirm that. The appearance of such: The bleeding comes at the times when the period is expected, but is usually without pain, or smell, and lasts for less days. It is also of different color. But many women (and doctors) still call it a period.
(Although we all know, a real period is impossible biologically).
How common is that?
This is, I think, different from implantation bleeding, since it mostly continues to three months and after that.