Thank you for your post.
I am unable to replicate the problem. This is what I did...
1. On my iMac running Mac OS X 10.6.6, I launched FileMaker Pro 10.0v3 and created a Text field, "myfield", and set it for four repetitions.
2. On my Layout, I added four fields, one for each repetition of "myfield".
3. I entered information into the first record into each of the four fields (repetition 1, repetition 2, repetition 3 and repetition 4).
4. I turned on Instant Web Publishing and set access for the file to all users.
5. On another machine, I launched a web browser, entered the IP Address of my Mac, received the Instant Web Publishing screen and selected the database file.
6. I was able to click in any of the four repetition fields for "myfield" without a problem, make changes, and submit the changes.
Please let me know what I'm doing differently than you, and I'll try to replicate the issue here.