Do not allow overwriting unspent transactions (BIP 30)
Do not allow overwriting unspent transactions (BIP 30)

Introduce the following network rule:
 * a block is not valid if it contains a transaction whose hash
   already exists in the block chain, unless all that transaction's
   outputs were already spent before said block.

Warning: this is effectively a network rule change, with potential
risk for forking the block chain. Leaving this unfixed carries the
same risk however, for attackers that can cause a reorganisation
in part of the network.

Thanks to Russell O'Connor and Ben Reeves.

wiggi/fairbrix-0.6.3,Crowndev/crowncoin,domob1812/crowncoin,Earlz/dobbscoin-source,Earlz/dobbscoin-source,Crowndev/crowncoin,syscoin/syscoin,wiggi/fairbrix-0.6.3,domob1812/crowncoin,domob1812/crowncoin,Earlz/dobbscoin-source,domob1812/crowncoin,Infernoman/crowncoin,Infernoman/crowncoin,domob1812/crowncoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,wiggi/fairbrix-0.6.3,Infernoman/crowncoin,dobbscoin/dobbscoin-source,Crowndev/crowncoin,Crowndev/crowncoin,dobbscoin/dobbscoin-source,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,dobbscoin/dobbscoin-source,syscoin/syscoin,domob1812/crowncoin,Infernoman/crowncoin,Crowndev/crowncoin,Infernoman/crowncoin,wiggi/fairbrix-0.6.3,Earlz/dobbscoin-source,dobbscoin/dobbscoin-source,Crowndev/crowncoin,Infernoman/crowncoin,Earlz/dobbscoin-source,syscoin/syscoin,wiggi/fairbrix-0.6.3,dobbscoin/dobbscoin-source,syscoin/syscoin,syscoin/syscoin