Cause

Two or more structure boxes with the same DT have the Main placement property.

Solution

In the Message management dialog, select the device tag assigned to the message. Use the Go to (graphic) and Go to 2nd coordinate popup menu items to consecutively navigate to the corresponding structure boxes in the schematic. When doing this, double-click the insertion points to call up the properties of the respective structure boxes. Deselect the Main placement check box of all structure boxes except one, so that only one structure box has this property. Repeat this process for all other structure boxes.

Then start a new check run.