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Feature Request: Duplicate Field Validation in ObjectBuilder

Feature Request: Duplicate Field Validation in ObjectBuilder: a task in LegoFlow-SWE (Harbor dataset). Problem: When building a Variant object using ObjectBuilder , inserting the same field name multiple times is silently accepted. No error or warning is raised, and the duplicate goes unnoticed…

The task

**Problem:** When building a Variant object using `ObjectBuilder`, inserting the same field name multiple times is silently accepted. No error or warning is raised, and the duplicate goes unnoticed until the data is read back, potentially causing data loss. This makes it impossible to detect mistakes early.

Part of Lego-X/LegoFlow-SWE.