TOPIC objects are unique. Hence the same topic object can't defined again. Topic objects are to be used for administration and topic strings for publish messages and subscribing to publications. As you are using the same DEPARTMENT
object name again to define a another topic, you are getting the error.
You can do it this way:
define TOPIC(DEPSUB1) TOPICSTR('DEPARTMENT/SUBJECT1')
define TOPIC(DEPSUB2) TOPICSTR('DEPARTMENT/SUBJECT2')
define TOPIC(DEPSUB3) TOPICSTR('DEPARTMENT/SUBJECT2/Minor1')
Later for receiving publications you can use the following sample topic strings.
"#" -> Receive all publications
"DEPARTMENT/#" -> Every publication under 'DEPARTMENT' topic
"DEPARTMENT/+/Minor1" -> All publications on 'Minor1' irrespective of SUBJECTs.