At times you will maintain or use a git repo which contains another repo as a submodule. Sometimes when you update or add this submodule again you may end up error like this.

'main_repo/sub_repo' already exists in the index.

To fix this run below command and try adding submodule again.

git rm -r --cached main_repo/sub_repo
git submodule add --force -b main {repo_url} {folder_to_use_for_sub_repo}