Duplicate labels can arise if commands run in parallel

[jonc125]

Probably a somewhat unlikely occurrence in normal practice, but doing smt run --executable date & ; smt run --executable date results in two records with the same label; subsequent commands like smt comment then fall over when given this label!

[apdavison]

This happens because the default label generator uses a timestamp accurate to one second.

There are three workarounds for this:

1. add sleep 1 between each smt run
2. set your own label explicitly for each run with smt run --label ...
3. change the label generator to create uuids, with smt configure --labelgenerator=uuid

We should definitely add a check for duplicate labels, however.