concatenate multiple attribute values and assign it to

I am trying to concat the value of multiple occurence of an attribute
output it as an attribute once.

<test node att1="1" att2="a" />
<test node
att1="2" att2="b" />
<test node att1="3" att2="c" />
<test node att1="4"
att2="d" />
 
I want to output this as
<testnode att1="1,2,3,4"
att2="a,b,c,d" />
 
How do you do this.
 
Thanks

Mohammed Parvez Shaikh
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