Request an official transcript from the institution where the credit was completed from. Send dual credit/college transcripts (if applicable) from the college via Speede (use code 003632 for TAMU), eScrip-Safe/National Student Clearinghouse/Parchment (use firstname.lastname@example.org if an email is required), mail, or hand deliver to Admissions.
It would be helpful to have a list of classes that you completed with you, or be able to pull up the unofficial transcript from the institution where the credit was completed in case there are any questions or prerequisite issues that come up during the registration process.
If you know you have already completed a course and received credit for the course, then do not register for that course at A&M. You can always confirm this with your academic advisor.