Ensure duplicated keys existence before validation

[Infinite-Null]

Trac Ticket: core 62424

Description

Enhances the validation logic for duplicated keys by adding proper existence checks before accessing array values. This prevents potential PHP notices/warnings for undefined array indices.

Changes

• Added isset() check before accessing $duplicated_keys array values
• Maintains strict comparison (===) with boolean true
• Preserves original conditional logic while making it more robust

Screenshot

[github-actions]

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[github-actions]

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