The Duality Query Engine leverages fully homomorphic encryption (FHE) and privacy-enhancing technologies to perform SQL-like queries on encrypted data, ensuring both query and results remain protected. It allows multiple organizations to collaborate on sensitive data without exposing raw data, enabling secure investigations and analytics while maintaining data sovereignty and compliance with global regulations such as GDPR, CCPA, and HIPAA. The engine supports fuzzy matching, access control, and dynamic collaboration parameters, and is optimized for high performance, executing thousands of queries per second. It is quantum-secure and integrates with existing infrastructure via RESTful APIs and Python SDKs, making it suitable for applications in financial crime detection, healthcare, and cross-border data analysis.