I'm not sure how long a "duplicate" private key would last. If someone with nefarious intents "happened" upon a private key which was in-use, then I suspect the coins would be transferred to an address solely under the bad person's control.

Conversely, an honest person would attempt to alert the person. So, I think, there are two questions. First, what legally happens to the thief, and second, what happens if the thief spends the coins?

Interestingly, a legal challenge to the introduction of Scottish Banknotes in 1695 can help us answer the later question.

At issue was whether the recipient of a tainted banknote could be held responsible for previous illegal acts committed with the banknote. Essentially, does the holder of a banknote (bitcoin) have free and clear title to that note despite its previous history?

The Royal Bank of Scotland prevailed in the lawsuit. The legal argument essentially said that not providing title upon acceptance of a banknote "would be to render the Notes absolutely useless, and consequently would in a great Measure deprive the Nation of the Benefit of the Banks, which could hardly subsist without the Circulation of their Notes".

I would suggest the same for bitcoin. That the holder of the private key must have free and clear title to the bitcoins represented by the private key.

Of course, just like paper currency, the acquisition of a private key by fraudulent means should be illegal.