Serial numbers are not always unique across an entire manufacturer. In many cases, they are only unique within a given model line, so two different camera models from the same brand can share the same number.

Manufacturers handle this differently:

• Some make serial numbers unique across all products or all camera lines.
• Others reuse the same number ranges for different models.
• Sometimes part of the serial number or the full product identification is used together with the model name to make it unambiguous.

So seeing the same serial number on a Nikon D3000 and a Nikon D5100 is not necessarily a problem. The important point is that the combination of model + serial number is what identifies the camera reliably.

Also, serial numbers are not always strictly sequential; some include internal coding or check digits to reduce entry errors.