Apparently I'd already installed the plugin some years ago, because upon installation there were already locations and workers filled in from a previous installation. During installation the input fields Email, Name and Phonenumber were added to the database and so now each were present twice due to the previous installation. I couldn't delete them via the plugins webinterface, so I had to resort to manually remove the database rows from the mysql database. This means that the uninstalltion several years back didn't clear and remove the database table correctly.
Maybe this is already fixed in the current version, otherwise you might want to check this.