Thanks for the help Arendonk and Horst,
I tried to debug more based on your suggestions :
API is not called multiple times.
during the process i found an alternate to fix the issue. however, i could not understand the cause of this
In my import mapping i used to get the object from input parameter and then pass it to object. when i tried to create a new object it works.
so alternative used is creating the object instead of using the existing one.