In a migration project from SharePoint 2007 to SharePoint 2013 the SharePoint Designer 2007 workflows are migrated correctly. The existing workflows keep working as they did in 2007.
However, when creating a new workflow things don't go so smoothly. First of all, you can't use the migrated 2007 list templates to create a 2013 list. You can easily solve this by saving a migrated list as a template and then use this list template to create a list.
I attached a SharePoint Designer 2013 workflow to this list. This workflow amongst others sends an email and changes some variables. However.. it does everything twice!
The solution/workaround for this problem is to add a 'pause for duration' in the beginning of your workflow. The minimum is 1 minute, but because the default value of the workflow timer job is 5 minutes it will take 5 minutes. You can change this value via the job definitions in central admin.