Yes it is a problem
bad idea. Next you're going to say you are going to use the same link for all accounts
Perhaps, but if IG are incredibly aggressive in anti-spam, it just means twitter are very aggressive in their anti spam
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The above are simplified answers, could their be ways to get around it, perhaps, but by the fact you are asking, it is probably better to go with the simply answers and not do this. Think of it like this, you get away with it for a few months and a couple of the accounts become profitable, making some $$ a day. One of the 15 accounts that never was successful gets banned and leads to your profitable accounts being banned. All because you didn't spend a couple of $ on extra proxies a month. That's the risk
1 account per IP is best and most safe
Everything down from there is worse and less safe on a sliding scale. It's a long way down to 15 accounts per IP