You can safely ignore these warnings. They are reported because the FFT is a block algorithm and is implemented in a serial loop engine fashion. Consequently, the data flow is not a continuous operation, and this leads to redundant arithmetic that is occurring constantly during simulation. Since different sections of the FFT only require some of the time, periodically there are X's in the redundant operations. Also, reading and writing at the same time is legitimate. Additionally, since simulation of a known good FFT generates these warnings, you can definitely safely ignore these messages.