OK, SAP APO is a separate system. Do I have to maintain the master data twice?
No. You create and change master data exclusively in SAP R/3. If you want to work with SAP APO, you need to transfer the master data from the SAP R/3 system. Depending on the scenario you want to use in SAP APO, you may have to enhance the transferred data with SAP APO specific settings, which are not relevant to SAP R/3.