If you are using an Applet with the "Add reminder to list" action from the iOS Reminders service, and the reminder is posting twice on your list, this is happening because you have IFTTT installed on more than one iOS device.
If you have, for example, an iPhone and an iPad that both have the iOS IFTTT app installed, both devices will carry out that action and post a reminder when the Applet runs.
To stop the reminder from posting twice, go into your settings to deny IFTTT access from Reminders on one of your iOS devices. When the Applet runs, it will only do so on one of your devices, and then iCloud will sync and the reminder will be posted across all of your devices.
Here is where to deny IFTTT access to Reminders: Settings > IFTTT > toggle off Reminders.