To be fair, that is the title of the topic. 
The window opening twice is strange. I tested my games and it seems to happen with newer builds (1.2.188) but not older builds (1.2.177) so maybe something was done at some point to make it happen as a side effect.
You could go through the releases and test older builds until you find one where it doesn’t happen? Then the code changes from that version could make it obvious of a possible cause.