Perhaps your tape recorder has auto reverse - which means it will play side 2 without you having to take the tape out and turn it over.
Stand your original tape and the tape you are recording onto upright as per this image
http://www.duplication.ca/jpg/C0-notches.jpg
You should see the original has no tabs and your tape will either have them in place or be covered with tape.
You say you have ruined tapes - hopefully you don't mean the originals! If its the tapes you are copying onto they are not ruined - you can record onto them again.