Does the Rapid Wrath magic weapon double a speed that's calculated by halving another speed?

Question

The specific magic weapon rapid wrath (Ghostwalk Campaign Setting 66) (11,702 gp; 3 lbs.), in part, says, "This +1 mighty cleaving shortspear doubles the speed of a creature that carries it" (link mine).

The crystal of aquatic action (lesser) (Magic Item Compendium 25) (1,000 gp; 0 lbs.), in part, says that when it's affixed to your armor you "gain a Swim [sic] speed equal to one-half your land speed."

When an adventurer possesses rapid wrath and affixes to his armor a crystal of aquatic action (lesser), is the adventurer's swim speed his land speed or half his land speed?

That is, does the adventurer, essentially, benefit from rapid wrath twice if the adventurer is lucky enough to have another movement mode's speed based on his land speed? Or is the adventurer already benefiting from rapid wrath because the adventurer's land speed was doubled to compute the new movement mode's speed, and his swim speed remains half his land speed?

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Note: As I type this, I (the DM) am leaning strongly toward the former—that is, doubling the newly acquired swim speed—, but the conscientious player whose PC carries rapid wrath and just acquired the crystal of aquatic action (lesser) thinks this is double-dipping and remains curious as to the community's opinion.

Answer 1

It is double-dipping. It also isn’t prohibited by the rules—they neglected to handle this circumstance. Which is really par for the course when it comes to creatures with multiple movement speeds, and movement speeds based on other speeds—a whole lot of the rules seem to forget that these aren’t things. (For example, can you tell me how far you can move if you walk to the shore, and then start swimming with your swim speed? I am rather certain this question has no answer under the rules.)

Anyway, I would handle this by assuaging the player’s concerns, saying you are perfectly content with this particular interaction and don’t have a problem with them “double dipping” this benefit to this swim speed. I would also make it clear to this player as well as all the others that this won’t be accepted as a precedent, and any other forms of double dipping along these lines has to be cleared with me, on an ad hoc basis.