Turns out the problem is quite easy to fix - it simply means that one of the fields you created in the list has a duplicate GUID (or you used one that is already in use by another field) OR you didn't add the the a field in the <FieldDefs> section.
Check the FieldDefs first and if they look OK, it is probably a GUID - simply go back through your Schema.xml file and check each one of the GUID's - be sure all of them are unique and that you DON'T change the GUID of the Title, LinkTitle or LinkTitleNoMenu fields if you are modifying them (i.e. like changing the display name). In one case I had to change each field until I found the offending one. Don't forget to use the MakeGuid utility (either through Visual Studio : Tools > Create GUID) or you can always download it.
FYI - Many of us end up changing the Title field - for this field, the following GUID's apply:
- Title | fa564e0f-0c70-4ab9-b863-0177e6ddd247
- LinkTitle | bc91a437-52e7-49e1-8c4e-4698904b2b6d
- LinkTitleNoMenu | 82642ec8-ef9b-478f-acf9-31f7d45fbc31