I can't think of a way to do it with processing steps alone, so I would handle it using two separate forms. For the first form (the one they actually fill out) I would include only the processing steps that occur regardless of whether they have already submitted. Include a processing step to add these prospects to a campaign. The campaign then sends them to a decision rule that checks if they have submitted the form within 24 hours. If they have not, they are sent to the form submit cloud connector to submit the second form, which is the same as the first form except that it includes all of the processing steps that you only want to fire if this is their first submission in 24 hours.
Make sense?
As a further backstop against redundant entries, you could add a 24-hour wait step to the campaign, so that they will be prevented from submitting the second form again within 24 hours.