The problem is that the coins are not in your wallet. Your coins are just a number in the blockchain. The wallet is only a file "password" to move your bitcoins from one part of the blockchain to another part of the blockchain.

If you loose your wallet file, the coins are not lost, they are still there. It's like having some coins of gold, and loosing the only key of a hyper secure safe box where their are stored in, while it is sinking in the Titanic. The gold coins are still there, but you will not be able to use them.

A wallet is a bad analogy. Let's try another worse analogy.

The blockchain is like a giant Google spreadsheet. Each cell has a password. With the password you can subtract a value from the cell and add the value to another cell. You don't need the password of the other cell where you add the number. The wallet file just store the password.

So you "have" 20 sheetcoins in in the cell G84 and the password is MHALL. (It's much more complicated. I'm ignoring a lot of technical details here.) So the sheetcoins are not in your wallet file, they are in the Google server. If you loose the password you just can't change your cell anymore.

Now let's suppose that someone decides to copy the spreadsheet to Office365. And they don't copy only the values but also the passwords. So now you "have" 20 sheetcoins in the cell G84 of in the Microsoft server. And you can move some of them to another cell to pay for something, because you have the password MHALL.

The coins are not in the wallet file, they are in the Google and Microsoft servers. So you have the "double" of coins.

If you instead copy the wallet file to a pendrive, or another computer, you don't double your number of coins. You are only copying the password to move them.

Now the problem of the two spreadsheets is that people must take them seriously. If nobody cares about the new copy in Office365, then your coins there are worthless, because nobody will accept a change in the Office365 version in exchange for something useful. But if everyone decides to
forget the old Google version and use only the new version, then the coins in the old version are worthless, they are just numbers. And there are some in between cases, were some people prefer one version an other people prefer the other version, and the coins in both version have some value. You have more the "double" of coins but the price of each will be lower, so you don't have the "double" of dollars.

One very important details is that the blockchain is decentralized, so you don't need a server in Google or Microsoft to host the spreadsheet. "Everyone" has a somewhat synchronized copy, and "everyone" process your request to move some coins from the cell G84 to another cell if you have the MHALL password.

(How can you send your password to everyone to prove you are allowed to use the G84 cell in a safe way, so no one can reuse your password to steal all your coins? Well, that one of the technical details I'm hiding.)

Now, since there is no central server and "everyone" host a copy of the spreadsheet, people can use a slightly different version of the spreadsheet hosting version. So some people will have the spreadsheet of version A and other will have the spreadsheet of version B. If a request to move coins is legal in version A and illegal in version B you will have a different value in the cell G84 in the computer of the people that runs version A and version B. It's easier to give a different name to each value, and say that you have some Acoins and some Bcoins.