Clarify how to handle redundant messages in ABA

[sbellem]

Currently, upon reception of an EST or AUX message a node will raise an error if the message (of the form (tag, round, value), e.g.: ('EST', 0, 0)) has already been received by the sender.

This issue is meant to clarify whether this is what the protocol implementation should really do, or whether it should simply silently discard the message and continue looping to receive messages.

[sbellem]

From a discussion with @amiller, for now we'll simply log the information regarding the redundant message and continue looping.

Will leave this issue open until the required changes are merged.