The type 'ProjectName.Web.YourType' already contains a definition for '_fieldName'

 

This is usually followed immediately by a second error (described the same way) except it shows the property name.  This seems to occur when a navigation link in EF is named exactly the same as the internal SQL Server column name.  For example, let's say you have a table with a field/column named ModifiedBy (represented as an int, and is a foreign key to another table), when the table is added to the EF model, it is represented as a navigation link.  If you decide to rename the link to "ModifiedBy", EF will allow this, but when RIA generates metadata for the client side, it will complain because, in the metadata, it internally creates both - the object for the navigational link as well as the internal type of the field.