Adds exist DOI validation when creating new DOI for DMP.

dmp-backend/web/src/main/java/eu/eudat/logic/managers/DataManagementPlanManager.java

@@ -977,6 +977,8 @@ public class DataManagementPlanManager {
             throw new Exception("User is not authorized to invoke this action");
         if (!dmp.getStatus().equals(DMP.DMPStatus.FINALISED.getValue()))
             throw new Exception("DMP is not finalized");
+        if (!dmp.getDoi().trim().isEmpty())
+            throw new Exception("DMP already has a DOI");
 
         // First step, post call to Zenodo, to create the entry.
         RestTemplate restTemplate = new RestTemplate();