I've been reading a little here and a little there. So far as I know its still called a theory.
However, I can speak from my own experience. Without ever hearing about dual factor theory I have been training according to its principles(I think) for the past 6 months. My memory and the charting of my bench press progress shows peaks and valleys like you would see with dual factor theory. Each subsequent peak higher than the last. Basically about this time last year is when I kinda got away from single factor and started training dual factor. My training curve the last 6 or 7 months shows peaks where I was at my strongest point. These peaks were accompanied with joint pain followed by a slight dip where I began to go back down in the amount of weight I could lift. I always consided this to be overtraining. What I have learned now is thats called overreaching. Its called overreaching because I never dropped below 10% weight of my best lift. Because I thought I was overtraining, I would step back for a couple weeks and drop total sets and even the amount of weight I was doing, and work myself back up again. Each time I worked myself back up, I would always start overreaching again, my joints would hurt and my strength would drop around 10% or less. The thing I noticed in my charting is each subsequent peak(near overreaching) was greater than the previous peak.
Here's my chart of my 1st set of benchpress over last 2 years(673 days). pay attention to the black circles. Those are points of nearly over reaching(this is not overreaching...yet). Its the slight dips where the true overreacing is occuring. The big drop down is my step back. The yellow circles are my step back points.
