When modifying the namespace , it is allowed to modify it to an existing namespaceshowname

[chenhao26-nineteen]

Describe the bug

When modifying the namespace name, it is allowed to modify it to an existing namespaceshowname, which is inconsistent with the logic of creating a namespace

修改namespace名称的时候，允许修改为已经存在的namespaceShowName，这与创建namespace的逻辑不符

Expected behavior
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Actually behavior
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How to Reproduce
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2. Click on '....'
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4. See errors

Desktop (please complete the following information):

• OS: [e.g. Centos]
• Version [e.g. nacos-server 1.3.1, nacos-client 1.3.1]
• Module [e.g. naming/config]
• SDK [e.g. original, spring-cloud-alibaba-nacos, dubbo]

Additional context
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[KomachiSion]

un reasonable, the namespace show name allow duplicated, only namespace id can't duplicate.