Distinct values in param text field

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How to see only the unique values for a text item which contains paramater values of an entity? On a page i can select a certain status. Then, the text item changes. But, when a status is already there, I don't want to update the text item, otherwise I get several records which are the same.  See image. There are mutiple records with status 'Open', I only want to see it once. I tried it with xpath in visibility settings, but no results yet. Perhaps with a MF?  
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The result in the page. When i hit the save button 'Opslaan', and the status is already given, he saves a new record with the same status. I just want to overwrite it, or a message which says that this status already exist. 

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The Parameter in the modeler

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