In the NV644, is the redundancy set by default for the Ethernet ports, or should it be configured to act this way ?
Does the redundancy mean that the Adtran will duplicate what comes from the T1 side and send it into both Ethernet ports in synchronization ?
They are configurable. You can leave one shut down, configure as different networks, put them on the same VLAN interface with spanning-tree for redundancy, or as a port-channel for redundancy and increased throughput (although with four T1s on the other side the increased throughput will be pointless).
When the same information comes from both Ethernet ports how is it handle and sent into the T1 side ?
This also depends on how the unit is configured. In a well-designed network, duplicate frames arriving on two Ethernet ports should not normally occur.