Windows mutex to prevent multiple instances

[Hans Nowak]

Samuel Bronson wrote:

>   Use a lock file, you could call it 'myapp.pid' and put the pid of your 
> app in it so it can serve double-duty.

We considered using a file to mark that the program is running, but dismissed 
the idea because of the problems this method caused in other apps we've seen... 
file doesn't get deleted, etc.

>   Obviously, you will need to have a path for it if you want it to 
> actually work.
>   You may also wish to consider why it is so important that only one 
> instance exist at a time.

It is important because the programs query the database, then write some stuff 
back; running it multiple times simultaneously can (and obviously will, as we 
have found) result in undesirable duplicate records.

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