Is this a re-load of the attachment? Was the same file or a file with the same name already attached to your page?
This error is usually given when attempting to upload an attachment by the same name. It’s as if the attachment is still present, but not visible on the list; the file must be renamed or uploaded in a different location to work around the issue.
Could you try that and see if it solves the problem, please.
In general, you can find all info about attachments (even so not about this issue) starting from this page of Atlassian’s Confluence documentation.
Please also check out our Video Tutorial Series “Learn Atlassian Confluence”.