No two choices should have the same ID at Microsoft.SharePoint.ApplicationPages.ChoiceComparerWithDefaultGroup.Compare
You probably get this error when you try to add an existing site column to a site content type.
The reason is that two site columns have the same internal name.
1. Check if the same error occurs at the root site when adding an existing site column to a site content type.
2. Once you’ve determined at which level the site column conflict is occurring, crack open your handy-dandy Sharepoint Manager 2010 (http://spm.codeplex.com).
3. Navigate to the fields (site columns) defined for that site and you’ll probably see an additional field that doesn’t show via the normal site columns page (_layouts/mngfield.aspx). This field is your problem field because its internal name matches the internal name of an inherited field.
4. Check that this field is not used in any site content types or lists. You may have to remove it from them first.
5. Delete the field with Sharepoint Manager 2007.