Jobs code updating value in custom field twice

[PV]

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Given a script in Jobs that updates the value of duration in a custom field by calculating since how many days the issue has been open. 

The calculated value is updated in the history According to crone expression like this: 

Duration : --null--   to 'less than 3 days'

But after this the duration is updating one more time simultaneously as follows: 

Duration: 'less than 3 days' to --null--

So the custom field value in edit shows null instead of less than 3 days.

Can anyone please tell any possible solution for this.

[Nic Brough -Adaptavist-]

You'll need to look at all your automations, jobs, listeners etc.  It sounds like the job you have set up is either wrong (maybe triggering twice and doing two writes for some reason), or something else is detecting the update and doing its own thing immediately.