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12.2 Project Navigator - Multiple instances of the same source can be opened
If you double click on a schematic or HDL source file in the Project Navigator hierarchy, the file opens docked within Project Navigator.
If you later double click on the same file, the existing open file is brought to focus.
However, if the open file is moved to a floating window (Window -> Float), double clicking on the source file will open a new instance of the source rather than bringing the floating window to focus.
In ISE Design Suite 12.3, Project Navigator has been updated to check for a source in a floating window before opening a new instance of a source file being opened.