bibliography issue

Dear Tex users,

I was trying to cite a reference in a paper which I was writing and finds
the below odd behaviour.
I use

at bottom of tex file


\bibliographystyle{plainnat}

\bibliography{draft_jet.bib}

.bib file contains entry in below format


@ARTICLE{Han1999,

author = {Han, Weiqing and McCreary, Julian P. and Anderson, D. L. T. and
Mariano,

Arthur J.},

title = {Dynamics of the Eastern Surface Jets in the Equatorial Indian
Ocean*},

journal = {J. Phys. Oceanogr.},

year = {1999},

volume = {29},

pages = {2191--2209},

number = {9},

month = sep,

__markedentry = {[sjo]},

file = {Han1999.pdf:Han1999.pdf:PDF}

}

input text

Ever since Kalus Wyrtki \cite{Wyrtki1973a} discovered the narrow jet

like current which flows eastward during inter-monsoon periods in

Indian Ocean, numerous observational\cite{Qiu2009} and modeling

efforts \\

\cite{O'Brien1974,Han1999} have been made to better

understand this unique feature of Indian Ocean. The surface currents

in the equatorial Indian Ocean reverse direction four times a year,


output text as seen

Ever since Kalus Wyrtki [Wyrtki(1973)] discovered the narrow jet like
current
which  ows eastward during inter-monsoon periods in Indian Ocean, numerous
observational[Qiu et al.(2009)Qiu, Li, and Yu] and modeling eorts
[O'Brien and Hurlburt(1974), Han et al.(1999)Han, McCreary, Anderson, and
Mariano]
have been made to better understand this unique feature of Indian Ocean.


I expected only Han et al. (1999) in output text but it outputs each names
along with the citation

Can any one suggest any solution?

-- 
with best regards

Sudheer
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