Cast Spark Double by paying ({3} magic symbol) ({U} magic symbol) , choosing to have it enter the battlefield as a copy of Enduring Scalelord with a +1/+1 counter on it.
Enduring Scalelord triggers, putting +1/+1 counter on itself.
Spark Double triggers, putting +1/+1 counter on itself.
Repeat from step 2.
Results
Infinite +1/+1 counters on Spark Double and Enduring Scalelord.