Hello Shihab,
Good catch. Despite the spec does not impose any constraint upon group `displayName`, we do so for our own reasons... So you are right and we should return 409 in this case
This [fix](https://github.com/GluuFederation/oxTrust/blob/master/server/src/main/java/org/gluu/oxtrust/service/scim2/interceptor/GroupWebServiceDecorator.java#L106-L106) has been included for our next release.