Users use custom Entry forms for Issue records. Users need to customize the Standard one and need to change the Issue Status using custom form and the notice occurs: Issue status must be duplicate to add a duplicate relationship.
The notice message could be specific only for used custom form or might be related to script or workflow.
The user can try to perform following steps:
1. Navigate to Customization > Forms > Entry Forms > set Standard form as Preferred (Check the Preferred checkbox to true next to Standard form).
2. Navigate to Lists > Support > Issues > find the Issue which has the problem and user will be able to change the status and save record successfully.
3. Then user can set the custom form as Preferred.
Note: In case the notice message occurs again, it is possible to edit the custom form, change the Name and select Save As. Mark new form as Preferred.
After changes users should be able to change the Status of Issue and save the record.