The IRS will not accept and process your tax return if someone else who already filed a return claimed any of your dependents. Provided you can prove that you are responsible for the individual in question, you can straighten out the situation with the IRS and receive the tax break that you're entitled to. The person who did this, however, could face IRS penalty for claiming a dependent she had no right to.
TL;DR (Too Long; Didn't Read)
Once a dependent has been claimed on a tax return, they cannot be claimed by any other individuals. If a single individual is claimed as a dependent by two different parties, it is quite likely that the IRS will launch an investigation to determine who is entitled to the dependent. If you believe that someone has wrongfully claimed your dependent, you should speak with an IRS representative as soon as possible.
Qualifying as a Child or Adult Dependent
For tax purposes in 2017 and 2018, a qualifying child is under age 17 at the end of the year, and is either your own child, whether natural or adopted, or a stepchild or foster child. A qualifying adult who is not a relative is someone who lived with you for the entirety of the year, did not earn more than $4,050 in gross income, and for whom you provided more than half of support. For relatives, a qualifying adult meets the same income and support restrictions, but does not have to live with you for all of the year. If more than one person provides support to an adult dependent, you must decide among yourselves who claims the dependent on the tax return. Multiple parties cannot claim the same dependent.
What if Someone Claimed My Dependent?
Sending tax returns to the IRS through the mail was once the standard way Americans filed their taxes. Today, more people file their returns electronically. Not only is electronic filing or “e-filing,” quicker, it allows you to catch problems that could result in filing errors or an incomplete return. If someone else has already filed a tax return for the year and claimed your dependent, the IRS won't accept your return when you attempt to e-file it. Instead, it will provide you with one of several error codes and explain the reason your return was rejected. If you file your return by mail, the IRS will hold the return without processing it and notify you of the problem via mail.
Fixing the Error on Your Return
When you claim a dependent, the IRS gives you credit for that dependent automatically and no one else can claim that person as a dependent for the year. Thus, if someone else already received credit for the individual, e-filing your return is no longer possible.
You must prepare and print out a paper copy of your tax return. Write a cover letter for the IRS explaining that you are filing by mail because someone else incorrectly claimed one of your dependents and explain how that person qualifies as a dependent. If, for example, you are claiming your grandmother, note that she lives in your home and you provide over half of her support. If you have any documentation supporting your claim, you can make copies and include the documentation with your cover letter.
Launching an IRS Investigation
When the IRS receives your return, it will evaluate your claims and compare your information to that of the individual who originally claimed your dependent. In most cases, the IRS will find and fix the error, and you do not have to take further action. The IRS will not tell you who originally claimed your dependent. If the other individual's claim was not a simple error, such as a typo when entering dependents' Social Security numbers, the IRS has the right to audit both you and the individual who originally made the claim to determine which of you is legally entitled to claim the dependent.
Other Important Considerations
Many situations which involve two people claiming the same dependent involve the children of divorced parents. After a divorce – especially in situations that involve joint custody arrangements – both parents may legitimately believe they have the right to claim the child as a dependent. Although the IRS has the right to audit both parents, it generally looks not at the amount of financial support each parent provided but at the amount of time the child lived with each parent. If the child lived an equal amount of time with each of his parents, the IRS will permit the parent with the higher adjusted gross income to claim the child as a dependent