why do my same webpage appear twice?

the same webpage appears twice in google SERP.

[echo1573]

my same webpage appear twice in google SERP,but their positions are different, i dont know why has they the phenomenon however some webpages have not?

any help is greatly appreciated!
thx

[thewebboy]

Probably once for "example.com" and again for "www.example.com"

[echo1573]

thank you very much!
but i have not submitted example.com,all my websites are the similar ,however i dont clear why some have and others not?i will appreciated if you give me some detail.

thx
Echo

[thewebboy]

Well, if you have a few links point to "example.com" google will go check it out, even if its a duplicate.

[lukey]

I have to say that I have seen this a number of times over the last couple of months. I get the *same* web page appear at 2 positions in the SERPS for a keyphrase.

It's def not a difference between www.widget.com and widget.com, checked that out thoroughly.

I think it's where the same page gets freshed and given a boost over and above the old one - somehow? ;)

[echo1573]

thanks!
if so,but i dont clear why the domain are the same in SERP,it ought to appear www.example.com and example.com.
please point out mistakes so that i correcte it.

thx