feat: deduplicate item at slice appending

[thxCode]

Add WithAppendSliceNonRepeated options, works like WithAppendSlice but deduplicate the repeat items.

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[sourcelevel-bot[bot]]

SourceLevel has finished reviewing this Pull Request and has found:

• 2 fixed issues! 🎉

But beware that this branch is 7 commits behind the imdario:master branch, and a review of an up to date branch would produce more accurate results.

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[darccio]

No new features will be added. It will be taken into consideration for v2.