Intermittent storeManagerStateSuite.TestStateWatcher test failure

[Ian Booth]

PANIC: megawatcher_internal_test.go:976: storeManagerStateSuite.TestStateWatcher

[LOG] 0:00.003 INFO juju.state opening state, mongo addresses: ["localhost:34872"]; entity ""
[LOG] 0:00.003 DEBUG juju.state dialing mongo
[LOG] 0:00.004 INFO juju.mongo dialled mongo successfully
[LOG] 0:00.004 DEBUG juju.state connection established
[LOG] 0:00.011 INFO juju.mongo dialled mongo successfully
[LOG] 0:00.014 INFO juju.state adding state server info to legacy environment
[LOG] 0:00.014 INFO juju.state found existing state servers []
[LOG] 0:00.021 INFO juju.state initializing environment
[LOG] 0:00.049 INFO juju.mongo dialled mongo successfully
[LOG] 0:01.079 INFO juju.testing reset successfully reset admin password
... Panic: {machine 2} mentioned twice in delta set (PC=0x4144D6)

/usr/lib/go/src/pkg/runtime/panic.c:248
  in panic
megawatcher_internal_test.go:1136
  in deltaMap
megawatcher_internal_test.go:1128
  in checkDeltasEqual
megawatcher_internal_test.go:1072
  in storeManagerStateSuite.TestStateWatcher
/usr/lib/go/src/pkg/runtime/asm_amd64.s:338
  in call16
OOPS: 0 passed, 1 PANICKED
--- FAIL: TestPackage (1.11 seconds)
FAIL
exit status 1

1.20.7 fix: https://github.com/juju/juju/pull/650