There are legit free (without capital investment) bitcoin giveaways even on twitter but it's not like the "send me btc and I'll double it" scheme by scammers.
This kind of scheme (the bolded part) is too obvious, but sadly there are lots of people falling victim to this. Greed sure knows how to make people lose their reasonings.
The only way to protect yourself from this is to use your head,
Should we headbutt them or what?
has anyone ever offered you to double your money for free on the real world and actually done so without issue? If the answer is no, then why would anyone expect this to happen on line and be a legit offer, it simple doesn't make a lot of sense.
People's greed dictates the mind and make it senseful. That is why those who wanted to earn more in a short duration of time often fall for scams.
Now, some may argue that when they saw the social media accounts of those famous and rich people do this they thought it was a legitimate offer but I am going to be very direct, you do not get to have so much money by giving it away without any restriction to people you do not know and in that case it is very unlikely the real owners were behind the messages and anyone using some common sense should have realized that within the first 5 seconds of seeing those messages.
Greed numbs people's mind. Whatever we say, it all goes down to uncontrolled greed. So we better control our greed and don't ever let it control us.