Uploading the same scanned survey twice would double-count responses and skew your results. PaperSurvey detects duplicates automatically using two methods that work independently.

Unique page identifiers

When unique identifiers are enabled, each printed page carries a unique page ID, a page number, and a survey ID encoded in its QR code. On upload, the system checks whether a page with those exact identifiers has already been processed. If it has, the page is flagged as a duplicate.

This catches both re-uploads of the same file and re-scans of the same physical page.

False positives

Legitimate pages can be flagged when multiple copies are printed from the same PDF, because they share the same identifiers. When this happens you can retry processing to treat the flagged pages as new responses, or enable "Allow duplicates" in survey settings to skip identifier checking entirely.

File hash comparison

Before processing any document, the system calculates a SHA-1 hash of each page and compares it against all previously uploaded pages. If an exact match is found, the page is blocked. This works for all surveys with no configuration needed.

Because the hash is based on the exact file data, re-scanning the same page produces a different hash. This method only catches identical digital copies, such as uploading the same PDF twice.

Reviewing flagged duplicates

Pages flagged by unique identifier checking appear on your survey's Uploads page marked as "Duplicate". You can review each one and choose to discard it or retry processing. File hash duplicates are blocked automatically and cannot be retried, since the pages are identical.