Try to install it with apt-get. Normally nodejs comes with debian 7, so installing it by compiling the sources may create a conflict like reported in the error. Follow the step from the doc:
This solution is ONLY for Debian 7, simply run the following as root :
$ echo "deb http://ftp.us.debian.org/debian wheezy-backports main" >> /etc/apt/sources.list
$ apt-get update
To install Node.js + NPM, run this :
$ apt-get install nodejs-legacy
$ curl --insecure https://www.npmjs.org/install.sh | bash