The API will automatically detect if your API key is creating the same image repeatedly within a short time period. When this happens, a new image will not be created and you will get back the URL to the existing image.
This protects you from accidentally using up your image quota due to a bug in a script or abuse from a user/robot.
We use the entire payload sent to the API to determine duplicates. If there a single character difference between images, it will not be considered a duplicate.