You can make a hidden free format question on same page in form of checkbox.
On first alert, along with showing error message, you can also set its value as 1.
While validating, check value of free format and question. if both are empty show error. If anyone have value let him go forward.
So, when first time respondent hit next button, both will be empty thus error message will pop up.
Next time, hidden question will have value 1 and he will be able to move forward.