Hi,
No, If you installed EHP6 for SAP ERP6, then no need to install SAP Netweaver separately.
SAP NetWeaver is SAP's technology platform. It is used to build and run SAP Applications and customer applications (Application server, data warehouse, portal, coding, messaging, etc).
ERP6 are products based on Netweaver Platform. That means SAP ERP runs on SAP NetWeaver technology platform and when you get SAP ERP6 you get SAP NetWeaver
You can check this in your ERP6 system
System -> Status -> SAP System data -> Component Information
Those are nothing but Netweaver components in ERP6 system.
I hope this helps,
Best regards,
Praveen