I have a text file with approx. 12 millions of lines, each composed of 4 fields (column 1, 2, 3 and 4)

Most lines have a unique STRING in column 2 and I would like to remove them. The entire text file is SORTED according column 2 (smaller to greater); lines with identical STRING in column 2 are always consecutive.

Spread in the text file, I regularly have 2, 3, 4 or 5 consecutive lines with the same STRING in column 2. These are the lines that I would like to keep in the final text file:

```
column1 column2 column3 column4  
WT 1 ILS G  
WT 2 DSG E          
WT 3 WYT S 
. . . .  
WT 106  AAA X  
WT 106  BBB Y  
. . . .  
WT 2704 CCC X  
WT 2704 DDD Y 
WT 2704 EEE X  
. . . .  
WT 87520 FFF X  
WT 87520 GGG X  
WT 87520 HHH X  
WT 87520 III Y  
. . . .  

```

What I would I like to achieve:

```
column1 column2 column3 column4  
WT 106  AAA X  
WT 106  BBB Y  
WT 2704 CCC X  
WT 2704 DDD Y 
WT 2704 EEE X  
WT 87520 FFF X  
WT 87520 GGG X  
WT 87520 HHH X  
WT 87520 III Y  

```

Thanks in advance for your time and help.
Best,

Laurent