Hi Tushar,
You have to remove the entry from RSSOURSYSTEM and then retry the steps.
Follow the below sap note and link
187479 - Assigning several source systems to the same ID
http://joeysbasisblog.blogspot.com/2013/01/connecting-two-bw-systems-to-same-ecc.html
With Regards
Ashutosh Chaturvedi