Well, there are a few HTML elements that you can use with being the standard for a quote and
being for a quote from another site or source.
With regards to duplication, I am not really sure I have ever come across a hard and fast rule here so tend to think the standard rules apply and if you have a good percentage of unique content on the page then having a few quotes won't matter. That is likely as true if you use italics, q or blockquote.
From my perspective, I would use blockquote to indicate it was from an external source and not worry about so long as 90% of my page was not quoted from an external source.
What you can't do is pinch content from another source and just wrap it up in blockquotes and call it good.
Seriously though, I quote things in my blog posts all the time so keep the unique ratio high and this is not something to worry about.
The HTML quote details can be found in more detail over at w3c schools:
http://www.w3schools.com/tags/tag_q.asp
If you google around this topic, you won't find much, a good indicator that people are not having duplication problems due to quotes and citations else the internet would be moaning about it.
Hope that helps.
Marcus