I think the fairest approach would be to have a clear one-submission, one-reward rule for sponsored activities. If the exact same content is submitted to two overlapping activities, it should only qualify for one sponsorship reward, while genuinely different submissions with the same theme, can qualify separately.
Hosts can make sponsored activities meaningfully different, for example, by requiring different formats, prompts, or creative elements. If two activities are too similar, the hosts should clarify which types of submissions are eligible before the challenge begins.
That would make the system more transparent and fair, clear rules, distinct activities, and no double rewards for the exact same work.