In addition to the technical and economic issues, there is a real concern that Government-Centric model similar to Data Retention will prove incompatible with GDPR and Human Rights article 8 as it requires government profiling citizen in order to detect attempted requests for data already collected for another purpose. This problem would not arise in Privacy by Design based Citizen-Centric Once-Only models. Citizen-centric models are under implementation in Denmark in a model named CitizenKey. The once-only principle can help to reduce administrative burdens in the EU Member States, as exchanging information that has already been collected is cheaper and less burdensome than collecting and storing it repeatedly. Furthermore, some experts believe that data protection concerns can be better taken into account. Process optimization and potentially higher administrative efficiency are described as main advantages of once-only for public administrations. These include fewer calls to customer service centers, fewer paper-based applications, faster processing of administrative processes, time savings due to reduced data capture requirements and fewer data errors due to data reuse. This could save costs for public administrations and improve the quality of various public services. Cross-border implementation of the principle could also help to ensure equal treatment for domestic and foreign persons and companies in the use of public services, for which they are obliged to provide information to public authorities. Better public value creation could, on the overall level, improve the legitimacy, transparency and accountability of public administrations. According to a study carried out on behalf of the European Commission, once-only encounters technical, organisational, semantic, and legal obstacles to implementation throughout the EU. On the technical side, there is a lack of a comprehensive and secure data exchange solution, interoperable catalogs and IT systems as well as a lack of infrastructure for a secure and data protection-compliant exchange. In addition, other barriers are organisational in nature. The study includes the lack of clear political guidance, the high implementation costs and the lack of administrative willingness to cooperate and exchange data. As far as semantic aspects are concerned, the challenge lies in the lack of or over-distributed registers and the insufficient interoperability of existing registers and data. In addition, barriers exist due to different standards, taxonomies, data models and data quality. Legal obstacles include the heterogeneity of national legal frameworks, data protection and respect for privacy. These aspects prevent a fast and far-reaching implementation of the once-only principle at European level. The European Data Protection Supervisor (EDPS), Giovanni Buttarelli, welcomed the EU-initiative, but claimed additional clarity on important data protection principles.