A dependent is not claimed for part of a year. It's either the entire year, or not at all. I would expect that for a 17 year old there is no way that 17 year old provided more than half of their own support in 2017. Therefore they are your dependent. The 17 year old will need to file an amended tax return to remove their self-exemption by selecting the box for "I can be claimed on someone else's return". When they do that, the 17 year old can expect to be returning some of their refunded money back to the IRS. Also, an amended return can not be e-filed. The IRS says so. They will have to print, sign (both the 1040 and 1040X) and mail it to the IRS. An amended return is processed manually by hand and can take as long as 16 weeks to process.
For you, if you will be claiming the 17 year old as a dependent on your tax return, you flat out can not e-file this year. That's because when the 17-year old e-filed with the self-exemption, the IRS computers have marked their SSN as "claimed". The 17 year old filing an amended return will not change that fact. So if you attempt to e-file your return with the dependent on it, your return will be rejected by the IRS. Therefore you have no choice but to print, sign and mail your own return too.
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