Schema Query re import and defines

Hi

Im not sure if I am mailing to the correct group or not.  We are having
a few problems with consistency with validation tools for XML schemas.
We have a main schema, extensions schema and an plug-in schema that
combines the two.  Descriptions on each are shown below:

Main Schema - contains data dictionary of all types, elements etc

Extensions Schema - contains business area specific types not contained
in the Main Schema - however these can be made up of types 'Imported'
from the main schema, therefore this schema imports from the Main Schema
(note that it uses IMPORT as it is a different namespace as the main
schema.

The plug-in schema - which 'Redefines' types from the Main schema and
'Imports' from the Extensions Schema.

One validation tool validates our schema's ok, however one tool says
that the item being redefined is declared twice (therefore fails
validation) - this is because the plug-in schema is indirectly importing
the definition (From the Main Schema)  via the Extensions schema, while
at the same time doing a redefine from the Main Schema.  What tool is
correct ? and is it valid what we are doing ?

Thanks

Simon.