It's pretty self explanatory. You have more than one route in your code that maps the route "/". Check your route settings for your controllers to make sure that they don't conflict with each other.
Did you make a razor/blazor project and copy/paste the index page, maybe even rename it, but forgot to change the `@page "/" ` directive at the top of the markup?
Newer Blazor is more helpful with its error messages:
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