Fix issue in duplicate mapping

components/device-mgt/org.wso2.carbon.device.mgt.core/src/main/java/org/wso2/carbon/device/mgt/core/service/GroupManagementProviderServiceImpl.java

@@ -616,8 +616,10 @@ public class GroupManagementProviderServiceImpl implements GroupManagementProvid
                 if (device == null) {
                     throw new DeviceNotFoundException("Device not found for id '" + deviceIdentifier.getId() + "'");
                 }
+                if (!this.groupDAO.isDeviceMappedToGroup(groupId, device.getId(), tenantId)){
                     this.groupDAO.addDevice(groupId, device.getId(), tenantId);
                 }
+            }
             GroupManagementDAOFactory.commitTransaction();
         } catch (DeviceManagementException e) {
             throw new GroupManagementException("Error occurred while retrieving device.", e);