as others have said, it is really rare. but, it is considered 'possible' to not know if you ovulate then based on the fact that it's more difficult to read the other fertility signals (cervical fluid and cervical placement). you can still check temps easily, but these other two are harder to check. Certianly, it's very difficult to detect the presence of cervical fluid with the presence of so much blood.
I have charted, on a number of occassions multiple ovulations during a cycle. two of these charts show clear 'double ovulations' with two discernable temperature spikes, the presence of cervical fluid, and the low and open placement of the cervix. a few others show 'false spikes.' I have cervical fluid and no temperature spikes, or temperature spikes and no fluid. I treat all of these are potential ovulation times--and i use barrier contraceptives during times when i know, think, or believe that i may be ovulating.
while it is rare for a women to ovulate during menstruation--for that matter to ovulate more than once a month (though not terribly uncommon for a women to not ovulate at all, but still have fertility signals except a strong temp spike)--it can and does happen. And because it's hard to 'read' it can be difficult to chart or track. in most FAM circles, it's considered appropriate to use contraception whenever you don't have a clear reading of all three signals. So, sex during menstruation includes other forms of contraception (or abstinance). this is the 'common' practice anyway.