$_=<>;
1 while (s/(\w)\s(\d+)(.*)(\s\1\s)(\d+)/$1." ".($2+$5).$3/e);
print

##</code><code>##

$ cat test
a 1 b 3 c 5 a 2
$ ./samekey test
a 3 b 3 c 5
$ cat anothertest
a 1 b 3 c 5 a 2 a 3 b 3 c 5
$ ./samekey anothertest
a 6 b 6 c 10