If you create a custom field of type select
Then add some values, try 3 as an example.
Save the field.
Then go back and edit it, say changing the drop down size from 3 to 1
Save the field
The save has repeated all the drop down values twice.
I had to go into SQl admin to remove the rows from the DB.
Confirmed.
Database Version: 4.1.22
PHP Version: 5.2.4
Web Server: Apache
VirtueMart 1.1 final
Joomla! Version: Joomla! 1.5.3
Submitted to tracker: http://dev.virtuemart.net/cb/issue/2050?navigation=true