Yes, it's possible.
In theory, the person spending it could choose which to spend. This is possible because outputs are referenced by block height & index so even if one-time public key is the same, you could specify one or the other. You won't be allowed to spend the same key twice, because it will have the same key image, but you could pick which one to spend. All other instances of the same key would be burned. In practice, the one picked by the wallet output-picking algorithm. Also, example of what can happen if key image check has a flaw.
(If this appears to be a vulnerability, requiring the TX-public-key (R) to be unique might resolve this)
That wouldn't resolve it. In fact, you could entirely omit the TX-public key but then you'd have to find some other way of communicating the shared secret to your recipient. Protocol enforces only what happens between outputs (one-time public keys). The sender could announce his TX key anywhere, it doesn't have to be the blockchain but it's a practical way of not losing it, and with it the possibility to recover the funds should the TX key be lost. To understand why, see Stealth addresses: where do they come from?
On a second thought, even if you copied existing P, the receiving wallet wouldn't even see it unless you copied the TX key R as well. The wallet could in theory spend it in place of the other, but in practice it wouldn't even see it. So yeah, requiring the TX key to be unique would resolve it but IMO it would be better to enforce outputs to be unique since we don't put any restriction on TX key R as it is not fundamental - it's just a secret message really.
