[BioC] calculating median expression between to identical gene names in data frame

[Srinivas Iyyer]

Dear group, 
I have a data frame with 180 rows and 25 coloumns
(data from a affy experiment).  I have 10 normals and
15 cancer lines. 

of 180 I have some duplicate gene names (arising from
two different probesets for a single gene). 

I want to calculate the median expression value for
these two identical genes. 

here I have 6 samples and 2 genes.
gids      N1	N2	N3	T1	T2	T3	T4
G1	10	12	11	40	46	39	46
G1	8	10	8	35	46	32	53

could any one please help me How can I calculate the
median expression for these two identical genes.
After I do this, I want to do clustering. 

thank you.

srini