Allow single record to have checkbox set

Hello, I would like to set an "active" status on a record using a checkbox field.  I only want a single record in the table to be active.  Does anyone know how to do this? I'd like to create a new record, set the checkbox, and have any previous record be unset.  Or at least throw a warning when the checkbox is set that there is another "active" record already.

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Mike Brady
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