Each record MUST have a unique "Device Name"! If you have a record called "MyPC001" and you wish to name another device to "MyPC001", it will fail. You must rename the device in stock to another name or delete the record prior to naming the device to MyPC001.
It is best to install the agent AFTER you set the final device name to ensure the GUID is configured using the final name of the device. Too much can go wrong if you name a device "NewBuild" and install the agent and then rename it to MyPC001. Sure this will work but then if you reinstall the agent for what ever reason to MyPC001 then the GUID would not be the same as the GUID created on the device initially installed and two records would then be created for the same device or you would not be able to merge the two records automatically into the same original record.
You may wish to contact support to see what may be going on in your instance of BCM and you may need to put a GUID or two or three into the banned GUID area of Global Settings > System Variables > Connection management Tab.