I'll answer both your posts in one:

Please show a sample.
Please show a sample.

Your sample data doesn't contain duplicates. This does: 

10.35.15.64/29     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.15.96/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.2.128/25     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.24.192/26    10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.24.48/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.25.128/25    10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.25.32/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.25.48/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.25.64/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.25.80/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.26.0/28      10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.25.32/28     10.133.3.11     OSPF-EXT1     3        if-illnevera
+nswerquestionsagain-10.133.3.11
10.35.26.16/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.26.192/26    10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.26.32/27     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.26.64/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.26.96/27     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.8.128/27     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.8.192/27     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.9.128/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11

[download]

If I run my snippet unaltered, I get 

10.35.25.32/28     10.133.3.11     OSPF-EXT1     3        if-servermgm
+t.cntx03-servermgmt-10.133.3.11
10.35.25.32/28     10.133.3.11     OSPF-EXT1     3        if-illnevera
+nswerquestionsagain-10.133.3.11

[download]

It outputs pairs: the previous and the current match. So if you have 3 lines starting with the same CIDR notated IP address, you'll get 4 lines back for that match. With 4 lines matching, you get 6 lines: 3 pairs.

<update> 

perl -ne '/^(\S+)/;if($S{$1}){print$S{$1}unless$s{$1}++;print}$S{$1}=$
+_' file

[download]

outputs just 4 lines for 4 matches.

</update>

--shmem 

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

In reply to Re^3: Searching for two lines that begin with the same string by shmem
in thread Searching for two lines that begin with the same string by tgrossner

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