Merge identical group colors

[WangAooa]

Fixes #6175

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at /~https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[calixtus]

Thanks for the update. Next time, please reuse the existing PR as otherwise we are loosing track of all the PRs. Please fix the remaining checkstyle issues, then we can review this.
Edit: On the other hand, this is already on review 😉

[tobiasdiez]

Thanks for your contribution.

Your solution looks good. It might be a bit more readable, however, if you instead use Stream.distinct in /~https://github.com/JabRef/jabref/pull/6491/files#diff-3550abce7f352ea1c73ed2c0e0e0dd37R181. Could you please try if this works indeed. I guess it would be good to add distinct also in the calculation of matchedGroupsString further below.

[calixtus]

Was closing this PR intentional?