400 Bad Request — Unique Property Violation

Message:
Cannot set property... value already exists on another record.

Cause:
Trying to assign a value (like unique_key) that is already used on another record. HubSpot enforces uniqueness for certain fields.

Fix:

• Update existing record Instead of creating it again in Hubspot;
• Either remove or reassign the value from the conflicting record.
• Set up filtering criteria and timestamps in the source system so only the required records are transferred to Hubspot. Also, ensure that all transfers from your source system to Hubspot run on a timestamp so you create only new records.