When new records are continuously added via API and you want to inject them into a journey, you should use Contact Evaluation set to Evaluate new records only. This ensures that only the newly added (not existing) records are evaluated for entry into the journey.
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Exact Extracts from Salesforce Documentation:
Source: Salesforce Help - Contact Evaluation Settings in Journey Builder
# "When using a Data Extension Entry Source, you can choose to evaluate new records only, ensuring that only fresh entries are processed." Source: Salesforce Help - Data Extension Entry Source
# "Select 'Evaluate new records only' to inject new rows added to the data extension into the journey."
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