Clone a resource in HTTP

In order to clone or copy a file, we execute a command like

cp a b

Given a file named a, the command creates a new file named b that has the same content as a. Similarly, we can clone a resource A. The client sends the following request to the server.

PUT /resources/B
...
{"source": "/resources/A"}

However, a client normally does not have the knowledge to properly name a new resource. A more reasonable alternative is the following request.

POST /resources
...
{"source": "/resources/A"}

All these are straightforward, but really smell like RPC. They did not follow the REST constraint of self-descriptive message. How could the server figure out /resources/A is what the client wants. What if the resource has been changed or deleted after the request was sent out?  The following approach may be better:

GET /resources/A

Then

POST /resources
...
the representation of /resources/A

What if the representation is huge? Maybe we can do the following:

GET /resources/A

or

HEAD /resources/A

Then

POST /resources
...
{"source": "/resources/A",
"ETag": "frompreviousrequest"}