Can you show us what the data looks like for a regular processed carton, versus a reprocessed carton? Are there any indicators other than a reused serial number?
Without seeing the data, I think it may be as simple as showing only the serial numbers that appear >1 time in whatever time period you're looking for.
for example:
| stats count by serial | where count > 1
From here, you could table or chart whichever fields you're interested in.
To answer your question about what is passed via the pipe: the results of the search before the pipe. If you're familiar with unix/linux command lines, it's the same principle.
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