I had the same problem happen to me today. I am running Firefox 34.0 (on Ubuntu 14.04). I only use the web client and only had 1 tab of it open. I am working at a cafe, which occasionally has spotty wifi. I did not have any of the other possible issues. I was uploading a document and Evernote allowed me to continue working on my note while the document was uploading. After about 5 minutes, it told me that the document could not be uploaded and then 96 copies of my note appeared.