This happens because Luminate API does not have any server-side prevention for duplicate API requests. Clients making use of the Luminate API to add functionality to their sites will need to add a logic using client side code to disable multiple submissions of the API calls; optionally they can also add a progress bar to warn the donor that the donation form has already been submitted.

The simplest form of client side code that could prevent multiple submission of API forms could take the form:
 
<input id="myID"  onclick="this.form.submit();this.value='Updated Submit Button Value'; this.disabled=true;">

The above piece of code is provided as a reference and there is no guarantee that it will work on your own API implementation.