You probably save the library under an other file name before importing it again.
Go to the folder where SH3F files are stored and remove the oldest SH3F file:
- under Windows Vista / 7, this folder is C:\Users\user\AppData\Roaming\eTeks\Sweet Home 3D\furniture,
- under Windows XP and previous versions of Windows, this folder is C:\Documents and Settings\user\Application Data\eTeks\Sweet Home 3D\furniture,
- under Mac OS X, it's the subfolder Library/Application Support/eTeks/Sweet Home 3D/furniture of your user folder,
- under Linux and other Unix, it's the subfolder .eteks/sweethome3d/furniture of your user folder.