insert select with on duplicate key

[Nopere Mike]

Hi,

In first sorry for my poor english,
I am on mysql 4.1.12
I have a little problem , i would like do this :

Insert into t1
select a,b,count(*) as d from t2
group by a
on duplicate key update c= c+d;

in fact , i would like insert into t1 all the t2 table , and if is exist update de c rows with the result of count(*) from table t2

If i can do this or if you have any idee for my probleme

Thx a lot ;)