fix(core): duplicate html title tags during ssg

[SoonIter]

Summary

https://github.com/web-infra-dev/rspress/issues/1164#issuecomment-2160793685

Related Issue

close #1164

Checklist

• [x] Tests updated (or not required).
• [x] Documentation updated (or not required).

[SoonIter]

the first commit is about the failed case

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