to the address 12iocUthp58E72ZksRmToDFPfM1WCPKv91. This is an address in my Bitcoin Core wallet, running on my laptop, for which I control the private key.
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The instant that this amount was received into the above address (before the transaction was even confirmed) the entire amount was then sent to another address 1aa5cmqmvQq8YQTEqcTmW7dfBNuFwgdCD which I have never heard and do not have the private key for
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What the hell happened and where is my Bitcoin?
You do not have exclusive control of the private key. Someone else has that private key as well.
How did you get that address and private key? Did you generate the address with the Bitcoin Core wallet immediately before sending the transaction? Did you import the private key into Bitcoin Core? Was it generated with VanityGen? Was is generated with bitaddress.org? Was it a "brainwallet", generated from a passphrase? Did you get the private key from someone else?
If you generated the address with the Bitcoin Core wallet, have you ever had that wallet.dat file installed on any other computer in the past?
The coins were transferred out the exact same second as they arrived, and before the transaction was confirmed, I did not think that was possible.
It is.
It is a good idea to wait until a transaction is confirmed before you spend the bitcoins that are received from the transactions (just in case the transaction never confirms), but it is not necessary to wait for confirmation. Unconfirmed bitcoins can be spent.