Custom Template - LISTBOX - ADDITIVE - Windows 2000

I am trying to create a custom template to add a custom dicitionary to
an office install the guts of what I have is:-

PART "Custom Dictionaries to Add" LISTBOX ADDITIVE VALUEPREFIX ""
KEYNAME "Software\Microsoft\Shared Tools\Proofing Tools\Custom
Dictionaries\"
END PART

However the 'ADDITIVE' directive seems not to prevent overwriting any
pre-exisitng settings in the key. I have tested this by applying it
using Local and (AD) Group Policy to teh same effect.

Once (just once) it seemed to work using Local Policy and on this
occasion the pre-existing setting appeared when the 'SHOW' button was
pressed.

Any sugestions about this welcomed.

Regards,