If both devices are the same, are configured the same, and respond the same, then it is impossible to tell if there is one, two, or four hundred.
However if there are differences between the two devices you will get corrupt data.
I2C is an open-drain protocol. For a device to send a 0 it pulls the data line low. For it to send a 1 it lets the pullup resistor pull the line high.
So if you have two devices and they both send a 1 you get a 1. If they both send a 0 you get a 0. But if one sends a 1 and the other sends a 0 then the 0 will "win" and you will see a 0.
Since most devices, when powered up, revert to some pre-defined default state, chances are you will always see multiple devices of the same type on a single address as just one device.
One possible way around it is to have some control signal that is unique to each device which will cause them to only respond one at a time. For example, you might choose to pull all the RESET pins of the slaves LOW and raise each one in turn to see what address the device is on. If you then see two that respond on the same address then you know you have a conflict and can report it as such.
Another possibility is to connect the address control pins to IO pins and communicate with all the slaves on the same address - but you only set one slave's address to the "live" address at a time. That also gives you the ability to have far more than just 8 slaves.
