Fix crash when search token are duplicated

[kean]

Fixes #22269.

Adds defensive code to make sure the search tokens never have duplicates. It should not be possible to add duplicate tags in the first place, but evidently, there are some sites that do have duplicates.

To test:

• (Regression testing) Verify that tags are displayed as suggested search token

Regression Notes

1. Potential unintended areas of impact: Post Search
2. What I did to test those areas of impact (or what existing automated tests I relied on): manual
3. What automated tests I added (or what prevented me from doing so): n/a

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

UI Changes testing checklist:

• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22275-3820cce
	Version	23.9
	Bundle ID	org.wordpress.alpha
	Commit	3820cce
	App Center Build	WPiOS - One-Offs #8308

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22275-3820cce
	Version	23.9
	Bundle ID	com.jetpack.alpha
	Commit	3820cce
	App Center Build	jetpack-installable-builds #7331

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[momo-ozawa]

LGTM w 1 comment

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

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[wpmobilebot]

	1 Warning
⚠️	This PR is assigned to the milestone 24.0. This milestone is due in less than 4 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

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