How strict is the definition of "native language" in PhD admission? I consider myself to have two native languages, including English; however, I'm not confident whether admission committees would consider this to be the case. One of the PhD programs of my interest requires all candidates whose native language is not English, including those who have obtained their Bachelor's degree in the U.S., to submit their TOEFL score. This requirement is stricter than that of other programs, and if I'm not considered to be a native English speaker, I would have to take a TOEFL for this program only.  

I'm 22 years old, and I have lived in the U.S. for the last six years. Until the age of 15, I had lived in an Asian country, where English-speaking population is small. However, since my parents spoke both their native language and English, I have been bilingual since my early childhood. I went to a public elementary school in my home country, and I graduated from a middle school, a high school and a college in the U.S., 
 in some of which I skipped grades.
You should contact the department directly, preferably by phone. I had a similar issue when applying for PhD, and the graduate admissions person was able to help me waive this requirement since he "could confirm that I am fluent by speaking to me on the phone". It all depends on the people you run into. But most of them are reasonable, so hopefully it will work out!