Let's say I flashback an Increasing Vengeance in order to copy a Shock twice. Then, I cast Reverberate from my hand targeting the Increasing Vengeance which in turn will also copy the Shock. Will the new copy of Increasing Vengeance make one copy of the Shock I target or two?
3 Answers
The new copy of Increasing Vengeance will only make one copy of the Shock - it was never cast, much less cast from your graveyard.
This isn't particularly simple - most of the time, when a spell or ability is copied, most of its characteristics are copied along with it (costs paid, choices made, etc.) - however, in this instance, since the evaluation of whether or not an extra copy is provided on resolution of Increasing Vengeance, that information is not actually copied. If it was phrased different it might actually work - something like "if the flashback cost was paid, copy twice." But phrased the way it is, you get only a single copy.
Relevant rule:
706.10. To copy a spell or activated ability means to put a copy of it onto the stack; a copy of a spell isn't cast and a copy of an activated ability isn't activated. A copy of a spell or ability copies both the characteristics of the spell or ability and all decisions made for it, including modes, targets, the value of X, and additional or alternative costs. (See rule 601, "Casting Spells.") Choices that are normally made on resolution are not copied. If an effect of the copy refers to objects used to pay its costs, it uses the objects used to pay the costs of the original spell or ability. A copy of a spell is owned by the player under whose control it was put on the stack. A copy of a spell or ability is controlled by the player under whose control it was put on the stack. A copy of a spell is itself a spell, even though it has no spell card associated with it. A copy of an ability is itself an ability.
You get one copy of the shark Shock* from the Increasing Vengeance.
Reasoning, by Comprehensive Rules, emphasis mine:
706.10. To copy a spell or activated ability means to put a copy of it onto the stack; a copy of a spell isn't cast and a copy of an activated ability isn't activated. A copy of a spell or ability copies both the characteristics of the spell or ability and all decisions made for it, including modes, targets, the value of X, and additional or alternative costs. (See rule 601, "Casting Spells.") Choices that are normally made on resolution are not copied. If an effect of the copy refers to objects used to pay its costs, it uses the objects used to pay the costs of the original spell or ability. A copy of a spell is owned by the player under whose control it was put on the stack. A copy of a spell or ability is controlled by the player under whose control it was put on the stack. A copy of a spell is itself a spell, even though it has no spell card associated with it. A copy of an ability is itself an ability.
Since the copy of the Increasing Vengeance wasn't cast, when the copy of Increasing Vengeance resolves, it will fail to see it was cast from the graveyard since it wasn't actually cast, it's a copy.
*Oops. Left original word struck out for entertainment value.
The answer can be divined from the second ruling on Gatherer for Reverberate:
When Reverberate resolves, it creates a copy of a spell. You control the copy. That copy is created on the stack, so it's not "cast." Abilities that trigger when a player casts a spell won't trigger. The copy will then resolve like a normal spell, after players get a chance to cast spells and activate abilities.
As the Reverberated Increasing Vengeance hasn't been "cast", it certainly hasn't been "cast from the graveyard"... and thus won't copy Shock twice!