Anyone can explain how work overtime??

I’m working as a PMO, and I usually sign in most of the day to get overtime. However, my seniority is low. I heard that there is a rule stating that if one person works overtime, they cannot work overtime again until others get a chance. I’m not sure how this rule works.

They gave overtime to the same person twice in one day. I raised this concern with the crewing team, and they explained that overtime is assigned based on timeframes, such as 7:00 AM–11:00 AM, 11:00 AM–3:00 PM, and so on. However, this does not make sense to me because we both have the same schedule and sign in for the same timeframes. Despite this, the same person is consistently given the overtime opportunity first.