Rearranged vector elements after std::unique

I'm currently working through Stanley Lippman's C++ Primer. In Chapter 10 generic algorithms are introduced.

As an example std::sort, std::unique and the std::vector member function erase shall be used to remove duplicate elements within a vector.

To see how a vectors elements are rearranged by std::unique I tried to print every element just to find that not all elements are printed. However a call to .size() tells that the size of the vector is as expected unchanged.

After compiling the program:

clang++ -std=c++11 -o elimDubs elimDubs.cc

and calling the programm with

./elimDubs the quick red fox jumps over the slow red turtle

The program prints

Size after std::unique: 10
fox jumps over quick red slow the turtle the  

which are only 9 of the 10 elements. (red is missing) Why? For the program it doesn't really matter as the subsequent call of erase is used to remove the duplicate elements anyways, still it irritates me that there are elements missing or at least not being printed.

#include <vector>
#include <string>
#include <iostream>
#include <algorithm>

void elimDubs( std::vector<std::string> &words )
{
  std::sort( words.begin(), words.end() );

  auto end_unique = std::unique( words.begin(), words.end() );


  std::cout << "Size after std::unique: "
            << words.size() << std::endl;

  for ( const auto &el : words )
    std::cout << el << " ";
  std::cout << std::endl;
}



int main(int argc, char **argv)
{
  std::vector<std::string> sentence;

  if ( argc < 2 )
    return -1;

  std::copy( argv + 1, argv + argc,
             std::back_inserter(sentence) );

  elimDubs( sentence );
}