This ensure that the appended record timestamp is greater than the same record consistentTimestamp (a record cannot self promote itself as consistent). The validation below ensure that the appended record consistentTimestamp is greater or equals than the most recently appended record consistentTimestamp (a consistentTimestamp cannot go backward). The combination of both check also ensure that an appended record cannot have a timestamp equals or less than an acknowledged consistentTimestamp.

I could have move the first check in a method of TimestampedRecord (a common superclass of Request and Response) but I prefered to put all the validation logic in one place. 

FYI here is what look like the LogRecord hierarchy:

LogRecord (id, consistentTimestamp)
 + TimestampedRecord (timestamp)
     + Request
     + Response
 + Index