Should I decline to fill out a recommendation form after saying that I will do it? I received an email from a student today. The email said that she was applying to a program, and that she needed her professors to fill out a recommendation form. She added that it shouldn't take long, because it should be easy to fill out. The deadline is exactly a week from today.

I am currently away from my institution, and will not return until her deadline, so this already puts me in a tight spot. She was also a weak student, maybe in the top half, but by no means a memorable one.

So I replied very quickly to her email (within 30 minutes of receiving her email), telling her that 1) if it's just the form, I can do it in a week; 2) if she needs a letter, I cannot write her a good letter within a week; and 3) overall, although if she insists I will do the job, she is advised to ask someone else for a letter.

She responded in a half-sentence, saying "it's just the form" So I asked her to send me her CV, transcript, and any relevant application materials. She has not responded yet.

I made a mistake here, though, because I was so busy that I did not open her attachment, thinking that if it's just a form, it should be easy to fill out. Now that I opened the form, I realized that I have seen this form before. I have already filled out this form for another student, and furthermore, I have written a detailed letter for her, because the form has an "additional comments" section that is very short (that I assumed had to be interpreted as a recommendation letter). I even asked about this letter here:
Should I write that a student is a feminist in my recommendation letter?

So now I am facing some tough choices.

Should I still agree to just fill out the form for her? Given that I have already written a strong letter of support for another student on top of the recommendation form, this will be a death sentence to this other student's application since her grades on the form will be lower, and she will not have a letter to supplement it (however, the first student is a lot stronger in every dimension that I am qualified to assess them on). 

I could email the student again, and tell her that because of the above reason, I cannot even fill out the form because it is too weak and that she will surely not get in.

Or, I could just do a half-assed job of writing a recommendation letter for this student as well, although the letter will have to be very generic and neutral, as I have none of her application materials, I do not remember her well, and I am also pressed for time since I am at a conference all week. Even physically sending the letter will be a pain since I do not get back until the day of the deadline.

So I am once again turning to you, hoping that I can get some quick answers (if I decline to write the letter, I had better let her know soon, so that she can find someone else). What should I do?
You seem really to be overthinking this. You're writing a recommendation -- you're not the student's full-time advocate or cheerleader. Fill in the form, send it in, be done. If you want, to mirror the structure of the other student's application, write a supplemental letter also for this one, just a paragraph or so -- shorter than what you've spent time on in your question here!

By the way: there is nothing at all wrong with having your recommendation for one student look worse than another's if you believe the former is actually worse for the program than the latter. In fact, I've often explicitly written on recommendations that A and B are both applying, and that A is stronger than B. Your job is to help provide an assessment with which the program can make a fair choice among applicants.