Weird. Looks like a pretty straight forward recipe. Although, I might have expected a bit more specialty grains. And with an expected OG around 1.062, I wouldn't expect that it just automatically needs extra time.
One thing that is significantly different than the way I did my last Dubbel is the sugar addition. Adding it 15 minutes before the end of the boil means two things; 1) it is getting cooked a bit, and 2) it is probably the first sugar that the yeast went after. If you didn't quite get the wort fully chilled before pitching, it is possible you just got a lot of 'hot' alcohols from the yeast working through that sugar super fast.
On my last Dubbel, I used candi syrup and didn't add it until 2 days into the fermentation. The yeast was able to work away at the malt based sugars for a while, and then the sugar was added, giving them a boost to attenuate fully.
The only other thing I can think of is that 68F is the bottom end of the suggested range for that yeast. I'm not familiar with that strain, so I can't tell you if that would create off flavors. Usually, lower than ideal temps don't do that, but who knows. You said the beer tastes like 'really bad candy'. Did you take a FG reading? Being at the low end of the temp range for the yeast, you might not have gotten full attenuation.