What is it :
Colloquially, it is a Diabolic way to win at athletic events.
Take a single one, and it might be shaped like a star.
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I believe the answer is
If you have two, they are
If you have one, it is
A steroid - ie. Asteroid, meaning starlike.
Starting the answer :
Diabolic way implies cheating and news reports always mention sports people caught taking anabolic steroids. It is usually, or "Colloquially", reported as simply "steriods".
Completing the answer :
Now take a single steriod, or "a steriod" to get "asteriod", which is shaped like a star !!!!