C++ - Is it better to use references in for loops?

Question

If I have something like this :
vector<string> v{"lorem", "ipsum"};

Is it better to do my loop like this :
for(string s : v){ ... }

or like this :
for(string &s : v){ ... }

So my question is, does this type of loop duplicate the data (and so it's better to use a reference) or not ?