It seems like you set both the report's and inner Data item'
s DataSource property.
Note, that if Data item is placed inside the Detail section it will be repeated for each record from its own data source and on top of that for each record from the report's(parent) data source.
To avoid this behavior set DataSource only for the Data item or reuse the parent's(report) DataSource - check an example in Binding a Data item to Data(Binding to data from the parent data item)
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