Warning! Doubling Slam Contracts can be Hazardous to your Score!
Now, first of all, there is such a thing as a Lightner double of slam bids. Some will tell you that this calls for the lead of dummy's first bid suit, while others will say, No, it calls for an unusual lead. The former has the advantage of being unambiguous and is probably best to stick with until you get real good. In any event, the point of the Lightner double is to induce the lead that beats the contract, the extra points accruing from the double being incidental. So, obviously, that's not what I'm referring to here.
I'm referring solely to doubles intended as penalty doubles, doubles based on the belief that the contract will be set and the desire to increase the penalty to be paid. This is very unwise unless you're looking at the A K of trump or are on lead with A K against 6 no. It is unwise largely because you stand to gain so little compared to what you might be losing. There are two basic dangers in doubling a suit slam: one is that they can run to -- and make -- a no trump slam, and the other is that you give away who has the key trump.
I have given some examples of this in No Trump Slams. In one, my partner, looking at the ace of diamonds and Q J 10 low in spades doubled 6 spades. Not vulnerable. We stood to pick up 100 points without the double and 300 with it, a sizable gain, one might say. But declarer ran to 6 no, which he could have made, but didn't. My partner had doubled this contract also, and so we picked up 100 points on the hand, the same score we would have had without telling declarer there's a better denomination for him to be playing the hand in.
But you know, even at the most basic level, the mathematics don't seem to be with you. If you double (not vul) a suit slam with two aces (as I recently saw), the increase for the double is only 50 points. But if declarer has a void (as she did on that hand), she picks 230 points! (if in a major). And that's without a redouble. With a redouble, that increase would have been a whopping, lemme see, 180 tripled plus 100 for making a redoubled contract, or 640 points! To pick up 50! That's more than a slam bonus not vul and close enough to a vul slam bonus to make you think a bit.
And that's without hitting the two really expensive doubles, i.e., where you send declarer to a makable no trump contract and where you announce a holding that induces a clever, slam-making line of play from declarer. Above are some links to illustrations of the all-too-common penchant.