Need assistance with awk/sed to identify/mark duplicate IP addresses

Good day. I have a text file which contains pod/node names and associated IPv6 addresses of which two pods have the same IP address, first pod **k8-worker0001c-cif-9d86d6dd4-vf9b9** and last pod **k8-worker0001c-ctdc-5bc95b699f-xnmrn**, the IP address being **2001:1890:e00f:3900::6**

$ cat /tmp/dup_ip.txt
k8-worker0001c-cif-9d86d6dd4-vf9b9
         2001:1890:e00f:3900::4/64 global nodad
         2001:1890:e00f:3900::6/64 global 

k8-worker0001c-cifpartner-64c89f8bc8-8p5pq
         2001:1890:e00f:3900::10/64 global 

k8-worker0001c-ctd-7d759784ff-2gk5d
         2001:1890:e00f:3900::a/64 global nodad
         2001:1890:e00f:3900::d/64 global 

k8-worker0001c-ctd-7d759784ff-hd8jp
         2001:1890:e00f:3900::c/64 global 

k8-worker0001c-ctd-7d759784ff-qkk4t
         2001:1890:e00f:3900::8/64 global nodad
         2001:1890:e00f:3900::f/64 global 

k8-worker0001c-ctd-7d759784ff-t6lwz
         2001:1890:e00f:3900::5/64 global 

k8-worker0001c-ctd-7d759784ff-vl8x9
         2001:1890:e00f:3900::9/64 global nodad
         2001:1890:e00f:3900::b/64 global 

k8-worker0001c-ctdc-5bc95b699f-xnmrn
         2001:1890:e00f:3900::7/64 global nodad
         2001:1890:e00f:3900::6/64 global

All I need is a one-liner to identify the duplicate IP address while retaining the rest, including the pod names. I have tried using **awk** **!seen** but that deletes the duplicate which I don't want. Therefore I'd like something like this:

$ cat /tmp/dup_ip.txt
k8-worker0001c-cif-9d86d6dd4-vf9b9
         2001:1890:e00f:3900::4/64 global nodad
         2001:1890:e00f:3900::6/64 global          DUPLICATE!

k8-worker0001c-cifpartner-64c89f8bc8-8p5pq
         2001:1890:e00f:3900::10/64 global 

k8-worker0001c-ctd-7d759784ff-2gk5d
         2001:1890:e00f:3900::a/64 global nodad
         2001:1890:e00f:3900::d/64 global 

k8-worker0001c-ctd-7d759784ff-hd8jp
         2001:1890:e00f:3900::c/64 global 

k8-worker0001c-ctd-7d759784ff-qkk4t
         2001:1890:e00f:3900::8/64 global nodad
         2001:1890:e00f:3900::f/64 global 

k8-worker0001c-ctd-7d759784ff-t6lwz
         2001:1890:e00f:3900::5/64 global 

k8-worker0001c-ctd-7d759784ff-vl8x9
         2001:1890:e00f:3900::9/64 global nodad
         2001:1890:e00f:3900::b/64 global 

k8-worker0001c-ctdc-5bc95b699f-xnmrn
         2001:1890:e00f:3900::7/64 global nodad
         2001:1890:e00f:3900::6/64 global         DUPLICATE!

Thanks in advance, Bjoern