It is caused by a bug of 2.0.9 or 2.0.9.1
I heard from onokazu that the unfortunately duplicated blocks are eliminated by overwriting these recent files int 2.0.10-CVS.
class/xoopsblock.php
http://cvs.sourceforge.net/viewcvs.py/*checkout*/xoops/xoops2/class/xoopsblock.php?rev=1.19.20.2/modules/system/admin/blocksadmin/blocksadmin.php
http://cvs.sourceforge.net/viewcvs.py/*checkout*/xoops/xoops2/modules/system/admin/blocksadmin/bl...After this, update the modules.