Dot graph language - how to make bidirectional edges automatically?

How about 'concentrate=true'?:

strict digraph graphName {
concentrate=true
A->B
B->A
}

From the documentation:

If true, use edge concentrators. This merges multiedges into a single edge and causes partially parallel edges to share part of their paths. The latter feature is not yet available outside of dot.

You should just use:

A -> B [dir=both]