I am a PhD holder in Analytical Chemistry from Iran and now, I am very interested in applying for a PhD program in a somewhat different field like Nanotechnology in the US and Canada. I have invested much time and effort on preparing for taking GRE and TOEFL exams and I have a strong research background, but now I heard recently from one of my professors that most institutions are not prepared to offer a PhD position to one who already has one PhD. Could you say more about this issue? Is this true?
Certainly, it happens. I am currently pursuing a second PhD myself, in a field very different from my first. I just learned that one of my fellow graduate students in our department also has an earlier PhD, and hers is from a field very close to our department's.
As the thread linked by ff524 notes, there are many people who wish to earn a second PhD for all the wrong reasons. But you didn't ask whether it was a good idea to earn a second PhD, you asked whether Universities would admit you. The answer is that many will, but they certainly may be wary of this aspect of your application. As commenters have pointed out below, some universities flatly will not consider your application. Amongst those that will consider your application, some admissions committees will not like your previous degree, and others will not care. In my case, I felt the admissions committee did not really care about my earlier degree; they cared about my ability to demonstrate potential in my new field.
Furthermore, before applying to my current institution, I emailed roughly eight departments asking whether an application someone such as myself would be welcomed. Every single one of them replied to me to say that yes, it would; and several were encouraging about my prospects if I did apply.
Nonetheless you should be prepared to accept that some admissions committees will not like your previous degree, and you should be prepared to explain to them your good reasons for pursuing a second PhD.
It depends. For example, in CS, some top schools don't allow this, like stanford MIT berkeley, etc. You should look carefully at the websites.