Chlorine dioxide will react with the chlorine in the water and remove it. This simply results in a little less chlorine dioxide in the water.
The residual chlorine in water is somewhere around 0.5 PPM. If you add 10 PPM chlorine dioxide, you will end up with just under 9 PPM after the reaction between the two.
Chlorine dioxide also reacts with iron, calcium, and magnesium in the water.