Tag Def creation api does not throw duplicate error when Tag Def with existing name is being created

API being used POST - service/tags/tagdefs

Case:
1. There is an existing tagDef with name as SSN with no attributes.
Payload -

{"name":"SSN","attributeDefs":[]} 

2. Now we are trying to create another tagDef with same name SSN but with attributes - name: expires_one, type: dateTime
Payload -

{"name":"SSN","attributeDefs":[{"name":"expires_on","type":"dateTime"}]} 

Actual Behaviour:
The api returns 200, but the 2nd tagDef is not created and existing tagDef is also not updated with attributes which are added in 2nd step.
Expected behaviour:
Since this is a POST api, an exception with duplicate object should be thrown indicating that a tagDef with name as SSN already exists.