Correlation set may be initialized only once

I was just trying a simple task to complete in BizTalk Orchestration.. i have created a custom property in my property schema and added the reference of that in the schema.
then assigned the value of the custom property in the message assignment shape like

Message_1 (mycustomproperty.customertotalamout) = 100;


i did this because i wanted to route my messages on the basis of the "customertotalamout" ..
everything is fine till this point.
then i added a
correlation set
Correlation_1

then when i tried to set the property at send port to follow the correlation set,
the compiler gave me following error

" Correlation set may be initialized only on".............

i had no idea what it was happening..
then i saw some post on net.. and figured out that i was doing this in a scope and there was already a global correlation set defined.
then
i defined the co correlationset_1 in the scope that i was using to send my message....

and then it worked........
Hope this will help someone..