[darcys22]

Single entry bookkeeping is essentially a list of your income/expenses. A list of this nature means you can't be certain you got everything, if an item is missing or has been double counted it wont be obvious at first glance that it is wrong.

Double Entry bookkeeping adds a layer to your lists to ensure that everything has been accounted for, you are creating the same list but also tracking the opposite side which is what it affected.

The most simple case here is that the income/expense also affects your bank account. By tracking the bank account you now can check that your "Predicted" bank balance according to your list is actually what is in the bank account. If they don't match then you know for certain something got missed.

It doesn't solve all the errors that can possibly occur, but it makes sure that the basics are at least covered.

[rendaw]

How does it make sure something hasn't been double counted? I assume you mean this situation:

1. Debit for X added 2. Credit for X added 3. Debit for X added again

Then you'll get a discrepancy when you sum them.

In a computerized system though you wouldn't make that mistake though, or else you could make it with double entry accounting too (just, you'll have double credit entries as well).

The only way to prevent duplicates is to associate upstream payment/transaction ids with records and make sure every entry is unique (which can be done for single entry accounting too).