Hi @Eyshika
Please delete the duplicate cuda .list file or entry in /etc/apt/sources.list
See my comment #11 in https://forums.developer.nvidia.com/t/the-repository-https-developer-download-nvidia-com-compute-cuda-repos-ubuntu1804-x86-64-release-is-not-signed/193764/11
Or the more comprehensive guide on GitHub: https://github.com/NVIDIA/cuda-repo-management/issues/4