1#diagram(debug: 0,
2  node((0,0), name: <A>)[$A$],
3  node((1,0), name: <B>)[$B$],
4  node((2,0), name: <C>)[$C$],
5  node((3,0), name: <D>)[$D$],
6  node((4,0), name: <E>)[$E$],
7  node((5,0), name: <F>)[$F$],
8  edge(<A>, "->", <B>, bend:  30deg),
9  edge(<A>, "<-", <B>, bend: -30deg),
10  edge(<B>, "->", <C>, bend:   0deg),
11  edge(<E>, "->", <F>, bend:  30deg),
12  edge(<E>, "<-", <F>, bend: -30deg),
13  edge(<D>, "->", <E>, bend:   0deg),
14  node(name: <G>, stroke: 1pt + black, enclose: (<A>, <B>, <C>)),
15)

Output: