Try adding a Trigger Condition. I tried this as it appears to work and doesn't trigger an infinite loop. This is Assuming that your UnitHeadText is formatted like a claims token (example: i:0#.f|membership|rickadmin@0v42t.onmicrosoft.com
@not(equals(triggerOutputs()?['body/UnitHead#Claims'], triggerOutputs()?['body/UnitHeadText']))

To avoid the infinite loop you must update the SharePoint item so that UnitHead Claims and UnitHeadText are equal before the flow completes.
Full flow:

SharePoint List values before flow runs:

SharePoint List values after flow runs:

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Cheers!
Rick Hurt