Update playlist import to only add duplicate playlist items sometimes

[PikachuEXE]

Pull Request Type

• Bugfix
• Feature Implementation
• Documentation
• Other

Related issue

closes #5038

Description

Updates the behaviour of importing playlists when existing playlist found

Before:

• Add playlist item unique by playlistItemId

After:

• If both existing & incoming playlists have unique videos only (i.e. unique videoId values)
  Only add playlist items from incoming playlist with videoId absent in existing playlist
• If either existing or incoming playlist has duplicate videos, add videos unique by playlistItemId (old behaviour)

Screenshots

Testing

• Download files from [Bug]: Videos duplicate themselves when importing playlists #5038 (comment)
• Clear playlist Favorites coz that's the name of playlist in those files
• Test following cases (skip some if you want, but A probably should not be skipped)

(A) Both existing & incoming playlists have unique videos only

• import PC1 twice
• Or manually add some videos (which is the case of [Bug]: Videos duplicate themselves when importing playlists #5038)

(B) Only incoming playlist has "duplicate" videos

• import PC1, import PC2

(C) Only existing playlist has "duplicate" videos

• import PC2, import PC1

(D) Only existing playlist has "duplicate" videos

• import PC2 twice (should still have 4 items coz unique by playlistItemId)

Desktop

• OS:
• OS Version:
• FreeTube version:

Additional context

[efb4f5ff-1298-471a-8973-3d47447115dc]

(A) Both existing & incoming playlists have unique videos only

Expected behavior: Not seeing any duplicates (This is the testcase from the issue)

(B) Only incoming playlist has "duplicate" videos

Expected behavior: You will see the duplicate because PC2 got it in its db

(C) Only existing playlist has "duplicate" videos

Expected behavior: You will see the duplicate because PC2 got it in its db

(D) Only existing playlist has "duplicate" videos

Expected behavior: You will see the duplicate because PC2 got it in its db

LGTM