In order to claim a child that has already filed a tax return, the child would have had to have filed without claiming themselves. If they used Turbotax to do their taxes, they would have had to indicate that "Someone else can claim me on their tax return" and if that is the case, fine just go ahead and claim them. However, if they did claim themselves and did not check the box next to "Someone else can claim me on their tax return", you have two options: ask the child to amend their tax return and check the box I just mentioned or do not claim the child on your tax return.
In order to claim a child that has already filed a tax return, the child would have had to have filed without claiming themselves. If they used Turbotax to do their taxes, they would have had to indicate that "Someone else can claim me on their tax return" and if that is the case, fine just go ahead and claim them. However, if they did claim themselves and did not check the box next to "Someone else can claim me on their tax return", you have two options: ask the child to amend their tax return and check the box I just mentioned or do not claim the child on your tax return.