Added unit test that will fail for duplicate entries.

refs #6760
Changed test method name. refs #6760
Remove duplicate log times before calling meanTime()

Not sure this is the best thing to do, ideally meanTime()
should be able to cope itself.

refs #6760
Added check for all times being the same. refs #6760

If the total duration of the differences of the log values
is 0.0 then just use the first log value.
Merge branch 'bugfix/6760_loadinst_duplicate_logs' into develop