non specific amplification in clone fragment using primer aganist vector

[muhammad yasir]

i have constructed 16s rRNA library in topo TA cloning vectore of invetroge and desigen primer for vector region near to the cloning site. but in the screening i am getting one light band that is extactly double (3kb) of the clone fragment (1.5Kb). i shall be very glad to know from some one, why i am getting this non specific amplification and how to solve this problem. 
 __________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com