If you have duplicate contact records which have the same email address for your contact in both records then as the email marketing has the 'dedupe' function set as default you will not be able to send out the same email more than once to the same email address.
If however you have duplicate contact records you may see the email marked as 'FAILED' inside the duplicate record - this is because the email has gone to to the 'master' contact record and the duplicate contact record has failed to be sent the email as the dedupe function is working .
Best practice would be to merge any duplicate contacts together to prevent this from happening.