I'm not talking about the 'adding insult to injury' kind of double whammy, like a putt cutting through the chains, spitting out the back, then landing on an edge and rolling 50 feet away. That sucks, to be sure, but when it happens, it happens. I'm talking about shot making strategy, where you plot out exactly what kind of shot you're planning to throw, with which disc, and how hard/soft/low/high, etc.
Sometimes, when we're thinking about our upcoming shot, we come up with an imperative that rises above everything else in our minds. For instance, we think "I need to do this to make sure I turn my disc over on this shot," or "I need to do this to make sure I clear that hazard." This kind of analysis is good, but when it isn't organized thought where a clear and detailed plan emerge, problems arise. Sometimes, instead of having a concrete idea of what we want to do, we let vague ideas marinate together right up until the disc is released. The result is what I call the disc golf double-whammy. Here's a good example that I've experienced many times:
I'm on the tee of a hole that is slightly uphill, with a dogleg to the left. Think DeLa hole 18 in the short left position. Being left-handed, I want to throw a backhand shot that turns over nearly the entire time. I consider the fact that uphill throws are harder to turn over, how long the hole is, and the placement of the trees. I know I have several ways to make a disc turn over more; I can throw it harder, put more spin on the disc, make the angle of release more exaggerated, make the flight path lower, or any combination of the four. The problem arises when I don't have a clear idea of which of these elements I want to use to craft my shot.
If my only notion is "I gotta turn this over" and I allow those four possibilities to mull in my mind right up until I release the disc, I may overcompensate to accomplish my imperative (in this case, "gotta turn it over"). So I might throw it much harder than normal AND exaggerate the angle AND, for good measure, my body might instinctively aim more to the left, resulting in a disc that turns over too much. Double (or in that case, I guess, Triple) Whammy.
Another good example I can think of is a classic. You have a disc you know is hard to throw straight very far before it begins to hyzer out, so you think of how to offset that tendency. So you end up aiming it wide AND putting turnover angle and power on it, then exclaim to your friends "I can't believe I just turned over that Excalibur!" or something like that. Classic double whammy.
The best way I can think of to combat double-whammy tendencies is to have a pre-shot routine that allows you to plan your shot in detail every time. Make sure your mind is nowhere but in that place, at that time, playing disc golf in a vacuum.