Are both devices using DLNA to access the My Cloud? If so you may need to check the Twonky media server interface on the My Cloud to see how it is treating both devices. Some users have had some strange behavior (including duplicate file names) if the Media Receiver Type on the Twonky Server Settings page is wrong.
One can typically access the Twonky interface of their My Cloud by using http://wdmycloud:9000 or http://<my cloud server name or IP address:9000> in their web browser.
Also the following discussion post has a lot of additional unofficial information on getting the most out of the My Cloud Twonky media server.
