If attempting to reupload an activity that failed to sync to Strava you may receive an error message that the activity is a duplicate. We're investigating an issue that's causing this error message even if the original activity was not successfully synced.
We hope to resolve this issue soon and we apologize for any inconvenience. In the meantime, please proceed with submitting a support ticket. Include the activity number that appears in the duplicate error message and we'll be able to delete it for you.
Follow this article for email updates on the issue.