# How Are Duplicate Uploads Detected? Source: PaperSurvey.io Help Center URL: https://www.papersurvey.io/help/recognition/article/how-duplicate-uploads-detected --- Uploading the same scanned survey twice would double-count responses and skew your results. PaperSurvey detects duplicates automatically using two methods that work independently. [Image: duplicate surveys] ## Unique page identifiers When [unique identifiers](/help/printing/article/unique-paper-survey-identifiers.md) are enabled, each printed page carries a unique page ID, a page number, and a survey ID encoded in its QR codes. 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. If your survey also collects responses through the web form, a paper upload whose response was already submitted online is flagged the same way to avoid double counting. ### 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. The uploaded file as a whole is also checked, so re-uploading a previously processed document is caught before any page is scanned. 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 in the Duplicates section of your survey's Uploads page. You can review each one, then either press "Process duplicates" to accept the selected pages as new responses or press "Mark Resolved" to note that you have reviewed them. File hash duplicates are blocked automatically and cannot be retried, since the pages are identical. --- Need more? The complete PaperSurvey.io help center and blog is available as a single document at https://www.papersurvey.io/llms-full.txt.