Is everything that happens in life predetermined? Why or why not?

I've been thinking lately if destiny really exists ir not, if destiny exists for example, in a contest scenario, no matter how I prepare or work hard for it, if another person is destined to win despite of not putting the same work as me, is it fair? Just because they're destined, they'd come off the best. Do you think destiny exists? Why or why not?
Is everything that happens in life predetermined? Why or why not?
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