That is what happens if you copy wallet files around.  If you copy your wallet file to a second computer, then they both think the money in the wallet is theirs.  If one spends any of it, the other doesn't know those coins are already spent and would try to spend them again, and that's the error you would hit.

Now that it's clear this is a key error message, it ought to be something more like "the money appears to be already spent...  this could happen if you used a copy of your wallet file on another computer."

You can move or backup your wallet file, but it needs to have only one "lineage" and only used in one place at a time.  Any time you transfer money out of it, then you must no longer use any previous copies.

This brings up a good point.  In the case of restoring a backup that may be from before you spent some coins, we need to add functionality to resync it to discover which coins have already been spent.  This would not be hard to do, it just hasn't been implemented yet.  I'll add it to the list.  This would make it mostly repair the situation instead of giving that error message.

The resync idea would go through your wallet and check it against the block index to find any transactions that your current computer doesn't realize are already spent.  That could happen if they were spent on another computer with a copy of the wallet file, or you had to restore the wallet to a backup from before they were spent.  Currently, the software just assumes it always knows whether its transactions are spent because it marks them spent in wallet.dat when it spends them.

A wallet merge tool is possible to implement but much less in demand once resync solves most of the problem.  With resync, you could do about the same thing by sending all the money from one wallet to the other.  The receiver would resync and discover all its overlapping coins were spent, then receive them in the new transaction.