Why are two diagrams the same instance? I hope they are different

GoJS
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1
1.jpg1440×1080 276 KB
2
2.jpg1440×1080 269 KB
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3.jpg1440×1080 184 KB
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4.jpg1440×1080 242 KB

when i use diagram.selection the first diagram.selection is null, the second diagram.selection can get a selected graphObject 。。。。why？？？