They explode twice. But only the explosion part, not the damage before the explosion. This is easiest to see with Scorch, the 734% damage from the trail gets buffed by Triumvirate + the 450% from Unstable Scepter, but not the double explosion part. And overlapping trails don’t stack. It would be much better if the damage stacked. The explosion part is very weak, at 221%. That’s the only part you get double explosions on.
So Scorch is basically a 221% Orb with double explosions, and then what is essentially a 734% non-stacking DoT.
The skill description for Spark is kinda misleading. IIRC, it zaps for half the tooltip damage, then lands and explodes for the other half the tooltip damage? That means the explosion part only does 174.5% WD. Very mediocre.
Obliteration and Arcane Orbit are pretty self-explanatory. There’s something weird with the Arcane Orbit mechanics and having multiple ones go off at once spamming the skill or something, when it was popular it actually was better in practice than it was on paper. I’ll just ignore that one for now, but know that even with the spam mechanics it’s still not as good as Frozen Orb.
Then the tooltip for Frozen Orb is pretty misleading too. There’s three components to Frozen Orb damage: the ice shards (315% WD), the projectile (635% WD), and the explosion itself (950% WD). The double explosion part only applies to the explosion damage. If you get it aimed right at the sweet spot, you can hit a single enemy with the shards, the projectile, and the explosion.
So with a double explosion, you’ll get 950*2 + 315 + 635 = 2850% WD.
Compare this to Obliteration, with a double explosion you get 700*2 = 1400% WD.
And with Scorch, 221*2 = 442% WD from the explosion with a non-stacking 734% DoT.
And with Spark: 174.5 + 174.5*2 = 523.5% WD.
Frozen Orb is just way stronger than all the other runes.
EDIT: That’s not to say you can’t fart around with Spark in T16 for funsies, but if you want to do GR100+, you’re probably going to want to go with Frozen Orb.