In case a Linked field contains 'ss' and 'ß' characters (within Query1 and Query2), Coresuite Designer considers the characters as equal and thus leads to duplication.
Reproducing the issue
1. Create a layout in layout designer, where both Query1 and Query2 produce Strasse and Straße in the results of the linked field (Feld1).
2. In layout designer, two AdvancedDataBands are set, while one is nested into the other. Both are linked to Query1 and Query2.
3. Preview the layout.
Each header line (Strasse and Straße) prints 2 records instead of 1.
Each header line (Strasse and Straße) prints 1 record.
Rendering engine limitation.
As a workaround, in layout designer set affected box (in this case Detail1) with the following Visible condition: adbData("Feld1") = advancedDataBand1("Feld1").