setPreventDuplicates(true) does not work

[zerozerosette]

Hello,
I have two listGrids and their corrisponding DataSource are defined as follow

1st DataSource

DataSourceTextField name = new DataSourceTextField("name");
DataSourceTextField fiscalCode = new DataSourceTextField("fiscalCode");
fiscalCode.setPrimaryKey(true);
setFields(name, fiscalCode);

2nd DataSource

setClientOnly(true);
DataSourceTextField name = new DataSourceTextField("name");
DataSourceTextField taxCode = new DataSourceTextField("fiscalCode");
taxCode.setPrimaryKey(true);
setFields(name, taxCode);

The first one gets its data from a gwt service, while the second one gets its own from the application, and so it uses setTestData().
Data transfering only works from the first one to the second grid: preventDuplicats works fine when the second grid is empty, while it fails (the record is added) when the grid contains some data.
Any hint?

Thank you very much