I'm not sure their exact origin. But I suspect they are mostly related to mining (PoS and/or
PoW) where transactions that were supposed to be deleted due to orphanage were not
deleted. I also suspect I had situations where I had two or more clients with copies of the
same wallet making PoS blocks simultaneously. There was also that period of time when
PoS mined blocks would get automatically orphaned because of the new trust rules automatically
orphaning PoS blocks that follow right after other PoS blocks. Whenever a transaction fails to
get deleted it becomes permanently inserted into the wallets cache of transactions. I discovered
you can erase all cached transactions without loosing the important stuff like the wallet keys and
account names. The block chain is after all the real master record of valid transactions.