No matter what household registration you have, if your employer pays social security for you, you do not need to buy rural basic medical insurance. The two cannot *exist.
If you have paid for the rural basic medical insurance first, and your unit has to sign a labor contract with you and pay social security, you need to cancel your rural basic medical insurance account first. When you go to China Construction Bank with your ID card and a letter of introduction from your employer, the staff will give you a phone call to help you cancel your original account. Then you can apply for a social security card, and then your employer can smoothly pay your social security.