Maybe, but not necessarily. The first person who claims a dependent prevents all succeeding claims with the same dependent's social security number from efiling- but they can still mail the return. The IRS processes the tax returns as they are received and do not hold refunds unless there are specific reasons. Usually if a dependent is claimed by more than one person, the IRS sends an official "friendly reminder" to both people who claimed that allows one to amend and/or provide the proof so that an official decision can be made. If you get this request, respond immediately.