Don't fail when opam return the same envirnonemt variable several times
Don't fail when opam return the same envirnonemt variable several times

Instead, print a warning and take the last bindings.

Fixes #46

dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,rdavison/jbuilder,rdavison/jbuilder