Thanks a lot for your answer and @Mrinal and confirming it would work with a pipe @tfrangio.
I was interested too in copying the meta values from one post to the other, so after creating the post with the duplicate with the pipe:
wp post get 6815 --field=content | wp post create - --post_title="Title of dup" --post_status='draft'
After I get the ID of the new post, I passed the meta_keys which I was interested...
14 is the source post_ID and
21 the target post_ID, I want to copy
for meta_key in tie_hide_related tie_sidebar_pos ; \
do wp post meta get 14 $meta_key | \
xargs wp post meta update 21 $meta_key \
xargs is needed to pass the value returned by post-meta-get to wp post-meta update, even if the documentation says it reads from stdin, without xargs some single values are read as an one item list, thus changing the behaviour of the value
Then, to copy the terms of a taxonomy, let's say
for cat_id in `wp post term list 14 category --format=ids` ; \
do wp post term add 21 category $cat_id --by=id ; done
This last step could be repeated for several taxonomies.