Question: How can I find two concatenated repeated lines in files?

For example, in this file we have only two concatenated repeated lines:

```
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05.ear
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter <--
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter <--
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05.xml

```

Answer: 
Uniq should be enough:

```
$ cat c.txt
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05.ear
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05.xml

$ uniq -D c.txt
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter

$ uniq c.txt
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05.ear
 OQ-63/ECC/Global/MES/CZ/adWerum-CZ-Adapter
 OQ-63/ECC/Global/MES/54/ECC-MRP-S05.xml

```

By default uniq checks adjacent lines of the input file. So for an unsorted file (like your case) uniq will do the job you want.

you might also be interested in uniq -d and -u option. See man page for more details (-d prints only one of the both duplicate lines , -u print only uniq lines - removes both duplicate entries).
Another option:

```
grep -zPo '\n(.+)\n\1\n'

```

This way we may add extra tuning (example accept extra spaces, etc)

Upgrade: as @thor pointed out this is not capturing repetitions at the begining of the file. To cover this situation use 

```
grep -zPo '(?<!.)(.+\n)\1' 

```