In my education documents and previous passport, I only had first name (NO last name).
Later I went thru name change process and applied a new passport with first name and last name.
Now I have got an ITA and while submitting the details, I have to enter any former names.
But while entering former name I notice that both first name and last name is mandatory.
We faced the same issue for my wife’s name.
We wrote a letter of explanation for “one and the same” name and mentioned that FirstName is same as “FirstName FirstName” and used the first name in all places as first name AND last name.