Comment 9 for bug 1703526

Digging back through one of the machine-0.log files where we saw this happening, we actually see the PingBatcher dying mid-way through which validates the theory:

2017-07-04 11:39:31 INFO juju.state.presence pingbatcher.go:119 ping batching loop failed: E11000 duplicate key error collection: presence.presence.pings index: _id_ dup key: { : "ae6dfe7a-e99d-45d9-8230-da097440e17f:1499168370" }
ca084bdd-5a88-41cf-8d67-f9a9dbd7dfd9: machine-1 2017-07-04 11:39:31 INFO juju.worker runner.go:483 stopped "pingbatcher", err: E11000 duplicate key error collection: presence.presence.pings index: _id_ dup key: { : "ae6dfe7a-e99d-45d9-8230-da097440e17f:1499168370" }
ca084bdd-5a88-41cf-8d67-f9a9dbd7dfd9: machine-1 2017-07-04 11:39:31 ERROR juju.worker runner.go:392 exited "pingbatcher": E11000 duplicate key error collection: presence.presence.pings index: _id_ dup key: { : "ae6dfe7a-e99d-45d9-8230-da097440e17f:1499168370" }