Same happened to me. I was able to remove the duplicate sources by deleting them from ~/.gnome2/gnome-dictionary/<source_name>.desktop.
Also to remove the unwanted Thai source that appears by default I renamed it at /usr/share/gdict-1.0/sources/<source_name>.desktop.
From some reason it appears to take the thai.desktop as the default source from /usr/share/gdict-1.0/sources
Same happened to me. I was able to remove the duplicate sources by deleting them from ~/.gnome2/ gnome-dictionar y/<source_ name>.desktop. gdict-1. 0/sources/ <source_ name>.desktop. gdict-1. 0/sources
Also to remove the unwanted Thai source that appears by default I renamed it at /usr/share/
From some reason it appears to take the thai.desktop as the default source from /usr/share/