Gun cameras did indeed exist in WW2 as you can check quite easily on the internet. Whilst there was inevitably some over counting the Luftwaffe did have quite strict rules and if anything double counting may have been less than with the allies. The number of kills in Spain was also not much of a factor. As joy ride says it is an interesting question. One comment I heard was that more Luftwaffe pilots were killed in accidents flying the 109 (narrow undercarriage/view out etc.) than were shot down.