title: pick up specific rows from duplicates on a dataframe with only selected columns
body: <p>This is a slight variation of the question that was answered previously in SO. 
  (<a href="https://stackoverflow.com/questions/9944816/unique-on-a-dataframe-with-only-selected-columns/26353097#26353097">Unique on a dataframe with only selected columns</a>)</p>
  
  <p>The only difference from that question and mine is that I have to mention which specific rows from the duplicates should be retained. My rows are names I am thinking something such as to give a substring to delete the rows which have that substring but I am unable to put it into codes. For eg: if duplicate rows are exm123 and tre123, I want to retain the ones with tre substring) </p>
  
  <p>If you guys think without any substring there are more easy ways to do the same in R, I am more than happy to learn the alternative. Thanks. </p>
  
  <pre><code>  dat:    
    Index Name      id1   id2
    1 exm-9980        1   202183358
    2 exm-53487       1   203186865
    3 exm-tre10248    1   85537661
    4 exm-7747       10   102827758
    5 exm-29639      10   18289634
    6 exm-76467      10   27436462
    7 exm-tre7540    10   18289634
    8 exm-4560589    10   74890584
    9 vg-194357      11   102589148
    10 exm-0867390   11   61110815
    11 exm-IN3127     1   85537661
    12 exm-tre2315   11   18632984
    13 exm-12411      6   30332555
    14 exm-128711    11   18632984
  
  nm1 &lt;- c('id1', 'id2')           
  indx &lt;- duplicated(dat[,nm1])|duplicated(dat[,nm1],fromLast=TRUE)    
  df22=dat[!indx|(indx &amp; grepl("^tre", dat$Name)),]    
  which(indx==T)       
  
  indx: 3,5,7,12.14,11,13        
  </code></pre>
  
  <p>when I cross check using values from id1 and id2 from the main data for index 13<br>
        f1=dat[dat$id1==6&amp; dat$id2==30332555,]<br>
  f1 is a matrix of 1 row. if it is a duplicate it should be a matrix of rows 2 or more. </p>
  
  <p>I am unable to load the full data as it is more than 100k rows. But I hope this will help in showing the problem in a clear way. </p>