Please leave my credits in the skript! I hope this is what you were looking for
The reason I made this and uploaded it is because I've had several box servers ask me to make this as they don't know how or a skript isn't easily available to do this (that I know of) I could be wrong and may be searching the wrong keywords but here's my shot at it!

/pvp off - turns off pvp
/pvp on - turns on pvp
/pvpblocks add - adds whatever block you're holding to the blocks that should give 2x rewards for having pvp on. (eg: If you had a coal mine you would add coal ore and/or a coal block)
/pvpblocks remove - removes whatever block you're holding and removes it from the blocks that should give 2x rewards. (The block needs to be apart of the pvpblocks list or it won't let you remove it)
/pvpblocks list - lists the current list of blocks that give 2x rewards when mined by someone with pvp on.
Dependencies:
Skript
SkBee