rustdoc: Prevent duplicated imports
Fixes#108163.
Interestingly enough, the AST is providing us an import for each corresponding item, even though the `Res` links to multiple ones each time, which leaded to the same import being duplicated.
So in this PR, I decided to prevent the add of the import before the clean pass. However, I originally took a different path by instead filtering after cleaning the path. You can see it [here](https://github.com/rust-lang/rust/compare/master...GuillaumeGomez:rust:fix-duplicated-imports?expand=1). Only the second commit differs.
I think this approach is better though, but at least we can compare both if we want.
The first commit adds the check for duplicated items in the rustdoc-json output as asked in #108163.
cc `@aDotInTheVoid`
r? `@notriddle`
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