I found out that a Nikon D3000 has the same serial number as another Nikon D5100.
I always thought serial numbers were unique at least among manufacturers.
Are they only unique amongst the same model body?
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All manufacturers* use a serial number that is unique only for a given model, not across the whole company - it's just that we don't compare our serial numbers with other people's very often so we don't notice.
Some serial numbers aren't sequential due to checksums to prevent keying errors (similar to the ISBN) but otherwise are still likely to be reused.
[*] I'm sure there will be exceptions but they'll be very rare
Some manufacturers choose to make each serial number unique across all model lines. Sometimes part of the serial number will be coded to indicate the model that serial number belongs to. Others choose to create serial numbers for each model line that may be duplicated by the serial numbers for another model made by that same manufacturer.