>I suggest you're correct that there should be some means of preventing the scenario you describe. The same user on the same workstation should not edit the same entity in different forms. I disagree that preventing a second instance of the form is the way to do that. Power to the user! When and if they navigate in the second instance to the same record, inform them.
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In our system we don't allow to open the same account by different users or the same user twice. However, we can open multiple instances of the same form for different accounts. There is potential problem as I described then two different accounts share some information and this iformation is edited. We're not refreshing forms automatically when data changes.