Run the following report for the month in which the cashbook shows the consolidated amount: View...General Ledger...Transactions...Entry Type Details, and then select the Entry Type (in the Start and End fields) for that cashbook's receipts.
The report should give a breakdown of the consolidated amount, by its reference.
I'm guessing that one of the amounts was allocated to the correct customer account, and the other amount to a different account, which is why you only see one amount on that customer account. The other possibility is that the other amount is reflecting on the correct customer account, but in a different period, and the common reference for the two amounts has grouped them together in one of the periods.
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