The file, which contains private details of Winehouse's death, was meant for her family's eyes only.
It was posted to them last week - but police got the address wrong, the Sunday Mirror reports.
They realized their mistake when an unrelated member of the public handed the papers in to a London police station on Friday.
A spokesman for Scotland Yard told the BBC:
"Police were informed that material relating to a forthcoming inquest may have been delivered to an incorrect address.
"Inquiries are now under way to establish the full circumstances of this matter."
Winehouse was found dead in her home in London in July.
An inquest into her death will begin this week. The cause of death remains unclear, says the NME; no illegal substances were found in her bloodstream, but her father has suggested that she may have overdosed on anti-anxiety drugs while attempting to detox from alcohol.
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