If your Azure Web Sites site is running on multiple instances and you create a new continuous WebJob, the WebJob will run on all instances by default. (Note that a triggered WebJob will run on just one instance selected at random.) You may want your continuous WebJob to run on only one instance. You can do so by creating a file called settings.job and adding the following to it:

{ "is_singleton": true } 

After you’ve added this line, save the settings.job file into the root folder for the continuous WebJob.

Alternatively, you can use the WebJobs API to do the same. There’s more information available on this at GitHub.