From ACT Wiki
The dual aspect concept that every accounting transaction has two sides.
Therefore the balance sheet should always remain in balance.
For example, if services are sold by a company for cash, the company's Sales figure increases AND its Cash increases.
Taking another example, if a company borrows money, its Cash increases AND its Liabilities (to repay the money in the future) also increase.
This system is sometimes known as 'double entry bookkeeping'.
2. Systems & controls - errors.
An error resulting from the inappropriate duplication - or inappropriate repetition - of an entry (or part of an entry) in a financial information system or elsewhere.