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It would be more helpful it you can provide a full stacktrace. It sounds like you have some anomaly in your catalog data...
We had the same issue. It looks like the issue was a result of the order of <Guid> and <Languages> node under catalog xml. It should be first and second child of <Catalog> respectively. The reason this error occurs is because how Mediachase code reads the xml stream. The import process expects <Guid> value to be first child of catalog and if it does not find in first child it assigns a new Guid and fails to process the catalog file as it would try to add a new catalog instead of overriding it. Moving the node orders solved the issues for us.
~ Sujit
When we call to Mediachase.Commerce.Catalog.ImportExport.Import method in order to import one catalog from an xml file , we are getting an Sql error regarding "Violation of Unique Key" but it shouldn't happen because we set the method param "overwrite" as true.
Full stacktrace:
Any comment or direction will be appreciated.