That has already been fixed, but the fix has not been released yet. It was first reported here: Missing break in AstarData.AddGraph()
It just logs that warning because you seem to be additively importing the “same” graph (as in that is based on the same original graph. Each graph get a unique ID when they are created, if you save one instance of that graph to a file and then load it additively twice then you would end up with two graphs with the same unique ID, which we obviously cannot have, so it generates a new ID). You do not need to worry about this.
Ah. Annoying that it happens after so long… Are you perhaps changing the width or depth of the grid graph during runtime?
Also. I could send you the latest development version so you could test that. If we are lucky it has already been fixed (as I have refactored a few things related to multithreading).