Google-forms – How to embed an HTML5 canvas in Google Forms

I've been able to find a solution for my problem. Using =ARRAYFORMULA(Sheet1!A1:A) in the top of a column Google Docs will update the whole column, without the need for functions in each cell. Thus, deleted or inserted rows will not cause trouble anymore.

I found this answer (by brettathds) at the Google Support forums.

You can apply unique id to each link you send out, using pre-filled links.

1. Add one more field to the form, e.g., "user token".

2. Follow the instructions for getting a pre-filled link, entering something like "qwerty" in the "token" field.

3. Inspect the pre-filled link that you get: it will look like https://docs.../viewform?entry.123456789=qwerty (there may be other entry.something parameters, corresponding to the field you left blank. You can delete those)

4. In the spreadsheet, append random-looking numbers instead of "qwerty". For example,

Column A: =floor((2+sin(sqrt(5)*pi()*row()))*1e9)

Column B: ="https://docs.../viewform?entry.123456789=" & A2

Here I used some math to quickly get a bunch of 10-digit integers that look random but are not.

5. Email these links to recipients; an Apps Script can help with this, if emails are entered in the same sheet.

6. After form submissions arrive, use vlookup or match to cross-check them against your list of token numbers. For example, if the Ids are in column F, then using =isna(match(F2, F$1:F1, 0)) in 2nd row and copying it down will show the repeated submissions as FALSE. Also, use match to check tokens against the column in which you computed them originally; this will detect any users who made up a token.