Yes. If you tax return has not been accepted by the IRS, it is not finalized. The rejected return must be changed to a format that is acceptable. This could mean removing a dependent, changing an EIN or SS# or any other reason it is not able to be transmitted.
If your asking if someone can file with a dependent that rightfully belongs to you because they were able to file before you, then yes, that can happen too. You would be required to mail in your return and the IRS would sort out who is actually able to claim the dependent.