$ cat -n config.log | head
     1  This file contains any messages produced by compilers while
     2  running configure, to aid debugging if configure makes a mistake.
     3
     4  It was created by configure, which was
     5  generated by GNU Autoconf 2.57.  Invocation command line was
     6
     7    $ ./configure 
     8
     9  ## --------- ##
    10  ## Platform. ##

# lines 3,6,8 are the duplicates in this little test.

# fields are tab seperated.

$ cat -n config.log | head |     # number the lines
   sort -k 2 |             # sort on second field 
   uniq -f 1 |             # uniq skipping first field
   sort -k 1,1n |         # numeric sort on first field
   cut -f2               # extract second field
This file contains any messages produced by compilers while
running configure, to aid debugging if configure makes a mistake.

It was created by configure, which was
generated by GNU Autoconf 2.57.  Invocation command line was
  $ ./configure 
## --------- ##
## Platform. ##