REST API SP.ContentType child content type creation error

Trying to create new content type via REST API in Sharepoint 2013, but I have hard time trying to provide "Parent" property. Sending to resource: /_api/Web/ContentTypes

{"__metadata":{"type":"SP.ContentType"},"Description":"TestDesc","Name":"ContentType New ONE","Group":"Custom Group","Parent":[{'__metadata':{'type':'SP.ContentType'},"Name":"ContentType ALREADY EXIST"}]}

What I have is: A duplicate content type "ContentType ALREADY EXIST" was found.

What if I want to make new Content Type as a child? I also tried X-HTTP-Method-Override:PATCH but as error states that SP.ConentTypeCollection doesn't support this method.

Now I have no idea how to create child content type for already created.