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Sprints do not necessary have unique names, so any Sprints can be configured with the name of an existing one, although this is not the default behavior.
I would check for the following possibilities:
1 - Check if the second Sprint of your board was not manually configured with the same name as the first one
2 - Sprints are not board-related, and will always be displayed in all the board that returns the issues under that sprint. That being said, if you edit your board filter to return issues from other projects that are configured with Sprints generated there, those Sprints will start displaying in your current board.
In order to display the issues in just one Sprint, you can:
1 - simply drag and drop the issues from one Sprint to another
2 - rename one Sprint to something else and bulk update the Sprint to the correct one, by creating a filter returning all of the issues in the wrong Sprint (With the name you added) and following the steps of this documentation to change it:
Let me know if this information helps.
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