When i tried to efile my federal, kept getting rejected so i mail it on the 15th. But efile just accepted federal return on the 17th? What will happen if i filed twice?

If your return was accepted through e-file and you also mailed it, the IRS will send you a notice when they receive your return in the mail.  You will need to contact the number provided on the notice and let them know what happened.  Unfortunately, there is nothing you can do preemptively to let them know, you just have to wait until they contact you.