Transaction reversal - I think

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Hi All,
I've had a spot of bother with Invoices being posted twice, and
forgetting to change the total payment when customers have added
something. I need to reverse the extra transactions (presumably?) -
could some kind soul explain how I do this (I'm using the default UK
chart, with VAT) - I'm affraid I go all blank on the accountancy side of
things, so words of one syllable would good.

Thanks in advance.

Cheers
Richard