error check for duplicate graph names

[yakra]

[17.7] ABORTING due to 1 errors:
1: Duplicate graph name Tourist in multisystem graphs

If a graph name is duplicated,

• single-threaded graph generation overwrites the 1st graph with the 2nd. The 1st has a DB entry, but no file.
• multithreaded graph generation attempts to write both graphs to the same file simultaneously, resulting in a corrupted file.

This prevents that from happening.
Ping @si404

[si404]

Thanks Eric, and welcome back.

Fixed the error locally. Pull coming in shortly

[jteresco]

An especially insidious error situation! I'll need to remember when talking about the dangers of concurrency in the Parallel Processing class in the fall.