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Monero payments are not guaranteed to be spendable, when individual transactions are scanned. Only when the entire chain is scanned will you learn if an output has conflicts, and only with the private spend key can you see if one of those conflicts has been spent, already having burnt all potentially usable outputs. This makes view-only scanning of the chain insecure, under the standing policy of "credit difference", as the wallet will credit the difference even if the funds are already burnt.
It should be noted wallet2 will stop crediting the difference if it knows the key image was used, as it should, yet it will continually credit if view-only, as it'll never learn when key images are used (unless imported, which may break a desired privacy model). Unfortunately, this is not an issue with a simple fix. Solely crediting the initial transfer potentially leaves fun