While working on BCMS for my organization I realized that there is a similarity between BC strategy and BC plan regarding the command center requirements (see 4.1.3 in BC strategy and 3.9 and BC plan). This makes me wonder where I will find such duplication and whether its necessary to fill in both documents.
Answer: There is indeed similarity between BC strategy and BC plan because the BC plan is a detailing of BC strategy when specific situations are considered (you can see this difference in the section one of both documents - Purpose, scope and users). For example, in BC strategy you may identify the type of database to be available for all BC plans, and in each BC plan you will detail where this database is located (e.g., server, physical address, etc.) for each business unit or site that specific BC plan covers.
So, you will find this type of duplication in every situation where you need to define high level guidelines and these must be implemented in different ways for each interested party. For example, in the BC strategy you define the general RTO (Recovery Time Objective) and RPO (Recovery Point Objective) to be achieved by the organization, and each business plan define its own RTO and RPO considering its own scenario (e.g., IT systems, financial processes, etc.) and the strategic RTO and RPO to be achieved.
Regarding if you need to fill both documents, the main criteria to consider this is the number of different conditions and BC plans you have. If you have many plans and each one of them has different conditions, it is better you have a BC strategy document to define high level conditions.