What happens if I file before my parents do I claimed as a dependent?

their returns would only be rejected if you indicated on your return that you could not be claimed as a dependent by another taxpayer. That little box. you don't get to choose whether or not you're a dependent. you are if you are either a qualifying child or a qualifying relative. 

you could be claimed as a qualifying child if all these tests are met:
• you have the same principal abode as the other party for more than ½ the tax year. Temporary absences like for school are ignored
• If not a full-time student, under 19 at the end of the tax year. If a full-time student under 24 at end of the tax year.
• you haven't provided over ½ your own support
• you didn't file a joint return unless there was no tax liability but merely filing jointly to facilitate refund of taxes withheld or estimates paid
or as you could be a qualifying relative if all these tests are met:
• your related to the other part(ies) or if not lived with the other party for the entire year
• your gross income for 2020 less than $4,300
• the other party provided over ½ your support
• you aren't a qualifying child of another taxpayer

also, your return could be in error if Form 8615 must be filed for because you meet all of the following tests: 

1. you had more than $2,200 of unearned income.

2. you're required to file a tax return.

3. you're any of the following:

a. under age 18 at the end of 2020,

b. you were 18 or over at the end of 2020 and didn’t have earned income that was more than half of your support or

c. you were a full-time student at least age 19 and under age 24 at the end of 2020 and didn’t have earned income that was more than half of the child's support. 

4. At least one of your parents was alive at the end of 2020.

5. you don’t file a joint return for 2020.

For these rules, the term “child” includes a legally adopted child and a stepchild. These rules apply whether or not the child is a dependent.