Read a article I think it was on Forbes website that a hard fork can mean some Bitcoin owners can unscrupulously double their Bitcoin balance? True or false?
It's nonsense.
Here's what that article as talking about:
You have 5 BTC before a hard fork.
Then a bunch of users and miners decide to implement a change to the consensus rules that that is not supported by the current versions of Bitcoin software. For the sake of making it possible to follow this conversation, lets call the new software NewBitcoin.
Meanwhile, some users and miners decide to keep on using the old software. For the sake of making it possible to follow this conversation, lets call the old software OldBitcoin.
So, now when a block is mined with the new consensus rule, the blockchain splits. Blocks that are mined by miners on the NewBitcoin chain pay NewBitcoins to that miner. Blocks that are mined by miners on the OldBitcoin chain pay OldBitcoins to that miner. NewBitcoins can't be seen on the OldBitcoin chain anymore than litecoins or dogecoins can be seen on the current bitcoin chain. You'd have to use an exchange to exchange NewBitcoins for OldBitcoins (or vice-versa).
Now, remember that you had 5 regular bitcoins BEFORE the fork happened. After the fork, you have both 5 NewBitcoins AND 5 OldBitcoins. You don't have 10 Bitcoins. Bitcoins don't exist anymore, because the Bitcoin software has changed. Instead there are two competing altcoins (NewBitcoin and OldBitcoins), and everyone that had bitcoins before the fork has that same amount of coins on both systems.
Of course, all of this requires that a significant number of users and miners continue to use the old software while a significant number of users and miners also choose to use the new software AND a block with the new consensus rules is mined.
More realistically, nobody is going to want to use the new software until the vast majority of users, merchants, exchanges, and miners have all agreed to use it. So, the remaining users of OldBitcoin will be such a small population, that at the time of the fork OldBitcoins will be practically worthless. The 5 NewBitcoins that you have would likely still be worth what they were worth as bitcoins before the fork, and the 5 OldBitcoins that you have would be worth a very small fraction of a penny. You could then sell off your 5 OldBitcoins to someone that still wants OldBitcoins for some reason at an exchange and pick up a few extra satoshi of NewBitcoin.
To make even this scenario very unlikely, the developers would almost certainly build in a trigger on activating the NewBitcoin fork. The new software would continue to operate exactly like the current bitcoin until there was confidence that a significant majority of users and miners were running the new software, then a trigger block would be set months in the future to give the minority that are still running the old software plenty of time to upgrade. Almost all merchants, miners, and exchanges would upgrade, because they would realize that OldBitcoins would be nearly worthless after the fork. There would be almost no merchants, exchanges, users, or miners remaining on the old software by the time the first forking block is mined, which would mean that there would be almost nobody wanting to buy OldBitcoins after the fork (since they'd have nowhere to use or exchange them). The resulting collapse in demand against a fixed supply (and deteriorating security due to lack of mining hash power) would turn OldBitcoin into just another silly worthless altcoin.
