title: join 2 lines only if field-1 are equals with sed or awk
body: <p>input file:</p>
  
  <pre><code>$ cat t.txt
  id1;value1_1
  id1;value1_2
  id2;value2_1
  id3;value3_1
  id4;value4_1
  id4;value4_2
  id5;value5_1
  </code></pre>
  
  <p>result would be:</p>
  
  <pre><code>id1;value1_1;id1;value1_2
  id3;value3_1
  id4;value4_1;id4;value4_2
  id5;value5_1
  </code></pre>
  
  <p>using sed or awk. Please give your opinion.</p>