[Python] FIND duplicates in Ansible Dictionary

I need to check that the same port isn't used in two sub-dictionaries.

my_dict:
first:
redis:
port: 1234
second:
redis:
port: 1235
third:
redis:
port: 1234


In a task I can use the following:

- debug:
msg: '{{ my_dict | dict2items | map(attribute="value.redis.port") | list }}'


This returns:

['1234', '1235', '1234']


What I need, is some way to filter only the duplicates. So I get:

['1234', '1234']


Returned.

Basically, I need to check that the ports aren't duplicated, and if they are apply a fail task that checks, and shows which items are causing the conflict.

Any ideas?

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