Fixes a bug with log replication:
If duplicate syncs get sent out and both succeed (as they should) this causes nextIndex/sendIndex to jump ahead.
This causes the subsequent sync to fail with the follower rejecting with a "local log is not up to date" error.
This in turn causes leader to resync by resetting  back its sendIndex/nextIndex to right after the followers last log index.
This also causes follower to timeout on leadership ping and initiate polls which get promptly rejected by leader.