Key defined twice

I had tried to use the git command to add the theme hugo-tranquilpeak-theme and whenever I tried to access my server, using hugo server -D, the following error occurred;

Error: While parsing config: (5, 1): The following key was defined twice: theme

What seems to be the problem?

Forgot to add, the terminal I’m using is Mac OS X.

can you post you git repo? impossible to say without any supporting code.

git submodule add https://github.com/kakawait/hugo-tranquilpeak-theme

I mean to YOUR repo.

Apologies. Still relatively new to the jargon; jumping straight from HTML to Hugo.

I was trying to add the theme based on the commands from the quickstart tutorial.

no worries, but to help we would need to see your config.toml at least, but better if your site can be looked through.

The error means you have theme defined twice in your config. Search for that, and delete the one(s) you are not using.