In case you are not following what lovinthehd means, here is an example:
The Emotive XPA-2 (original generation) manufacturer's specs claim
250Watts RMS into 8 Ohms and
500 into 4 ohms.
When Gene measured, he stated:
http://www.audioholics.com/amplifier-reviews/emotiva-xpa-2/xpa-2-measurements
So, instead of two times the power it actually achieves
1.64 times the power (512/312=1.64)
So, what we see is Emotiva decided they wanted to claim "double down" status more than claiming the additional 50 Watts per channel into 8 ohms.
It is a marketing decision based on what they think will sell better.
Aragon did the same with their 8088:
They claimed 200/400 for 8ohm/4ohm, but tests showed 245.9/408.1 for 8/4.
http://www.stereophile.com/content/aragon-8008-power-amplifier-measurements#toX9c8DVeQ6kGLfS.97
So, again, instead of doubling down, it is actually
1.66 times the power when the impedance is dropped to 4 ohms.
Truly doubling down is a bit of a "Holy Grail". I don't know which if any amps truly do it, but if any do, it would reflect exceptional circuit design with exceptionally efficient current handling capability.
However, as has been mentioned, it really doesn't matter unless your speakers are low impedance! And even then it is really about maximum power at 4 ohms (or whatever is the impedance of your speakers), not whether it doubles down.