Complex sentence: Ignoring this aspect is one of the most common mistakes made in calculating RPI, for, mathematically, it shouldn't be there, and the most re-occuring opponent of your opponents is yourself.
Split into simpler sentences: Ignoring this aspect is one of the most common mistakes made in calculating RPI. The reason for this is because, mathematically, it shouldn't be there and the most re-occuring opponent of your opponents is yourself.