I don't think mmap stops you from mapping the same memory twice, nor does linux. In my case I didn't even disable the system I2C driver. If course if I tried to use it I would get io errors and if I am not careful I can crash the kernel, but that's what you get for poking around. This was a raspberry pi.

Yeah, that's what I meant since i2c is a serial protocol and most drivers assume mutual exclusion on device registers.