Check for duplicate items

[koppor]

Kind of follow-up of #402

When contributors merge the CHANGELOG.md file wrong, then we have duplicate entries - one for a released version and one in the unreleased section.

Example - following appeared twice in our CHANGELOG.md

- We added support for Cygwin-file paths on a Windows Operating System. [#13274](https://github.com/JabRef/jabref/issues/13274)

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Triggered by JabRef/jabref#13535