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