I started to read this artcle and I noticed there are 2 が particles in the title
If the first が marks the 2000+ children as the subject, what is the purpose of the second が?
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This is not a double が because each occurrence actually belongs to a separate clause.
The first が indeed tells us that the subject of the main clause is ２０００人以上の子ども.
However, the second が belongs to the phrase インターネットが原因で "As a result of the Internet", and tells us that the subject of that phrase is インターネット "the Internet".
Note that by omitting the phrase インターネットが原因で,
we see that the true subject of the main clause is ２０００人以上の子ども.