If Android discovers songs or media twice ...
This happens mostly after changing the SD-contents via USB and seems to be a sync problem happening randomly, while seldomly, after ejecting the USB-device.
To fix this, clear the data and cache of your media service app and restart the device. Do not however delete data or the cache of you music/media app, since this will remove settings, playlists and so forth. You can still try that afterwards, if the clearing of the service didn't get everything.
You can find the app data in
system settings > apps > all > media storage