After importing an article into AEM from the AEM Mobile On-Demand portal and making an edit to the metadata, attempting to reupload the article fails with the error "Article <article name> already exists in AEM Mobile On-Demand".
Cause is uncertain. Possible version number conflict is one of the causes. Manual editing of the article properties, especially the DPS ID, in JCR also causes this failure mode.
Ensure that you have imported the most recent version of the file into AEM from AEM Mobile. Then remove the article from AEM Mobile, make the edits in AEM, and reupload to AEM Mobile.