I agree that the return would be rejected if a return had already been filed for that SSN.
However, there is something new this year for dependents. If the primary taxpayer has an IPPIN and claims a dependent that has already been claimed on another return, the return will be accepted. The dependency will still have to be resolved and the taxpayer should expect a notice, but they do not have to paper file.
If the primary taxpayer does not have an IPPIN, the return will be rejected as in prior years. (If they want to e-file, they can obtain an IPPIN and re-submit.)