Validate against duplicate or otherwise corrupt content

A lurking bug for sure, but one that would likely cause issues for us elsewhere too. The docs site would probably struggle if duplicates existed. We could benefit from a "validate" mode in this tool that would report on any such issues; we could then define the behavior for invalid content.

_Originally posted by @jeffhandley in https://github.com/carlossanlop/DocsPortingTool/pull/77#discussion_r681089559_

该 issue 没有指明任何文件、测试或入口点。首先定位工具的内容处理和命令行模式处理，然后确定当前如何处理重复或损坏的内容。完成的标准是 validate 模式报告这些情况，并定义对无效内容的处理行为。

由索引模型根据 Issue 内容生成。