Although it says on the "Edit preferences" page that one has to enter one's password twice only if one wants to change one's password, I find that I have to enter my password twice to make any additions or changes.

Impossible.
Check once again (for example, on our site here).

Ok. This is what I just did, using Internet Explorer 5 on Windows 98 on this forum.

I registered as: (username) newtest with (password) testnew. I filled in just the required fields. I then signed in, went to edit preferences where I filled in the occupation field and entered my password once, on submitting the form I got the message "Error in user data: Passwords didn't match!"

I then filled in the second password field, submitted the form and got the message, "Preferences updated, password changed, re-login on the main page".

You don't need to fill out neither first, neiter second password field if you are don't changing it.

Ah Ha! Though password is still starred as a necessary field when one is editing preferences. So my mistake is understandable.