[5.4] warn when duplicate subform fields are removed on save

[krishnagandhicode]

Pull Request resolves #33327

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

This fixes the silent duplicate filed removal behavior for Subform fields

this fix now : on saving a Subform field, check for duplicate selected custom fields in the options list, if found any the duplicates are removed during during save (existing behavior), but now a warning message is shown to the user so the change is not silent and the user is aware of it.

Testing Instructions

Navigate to : Administrator > fields
create a field only give it a title and save and close
now create a new field or type subform (i.e select Subform (subform) in type field) > now go to Fields (at end in General tab) select the same custom field more than once > Save & close
(see the video in Before and after Patch section in this disc. if unclear)

Actual result BEFORE applying this Pull Request

Duplicate subform fields will be removed on save without any warning,
so after save when you check your subform field each custom field appears only once. (but the user is unaware why)

Before.mp4

Expected result AFTER applying this Pull Request

Duplicate subform fields are still cleaned up on save, but users now receive a warning message saying that duplictes were removed and that each field can only be used once

After.mp4

Link to documentations

Please select:

• Documentation link for guide.joomla.org:

• No documentation changes for guide.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[exlemor]

I have tested this item ✅ successfully on 6055428

I have tested this successfully - thanks @krishnagandhicode

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[SRV-KILLER09]

I have tested this item ✅ successfully on 6055428

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[richard67]

RTC

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[muhme]

✅ Final test before merge with JBT

• Seen silent deletion of duplicate fields in subform field
• Applied PR with Patch Tester
  • Seen yellow warning 'Duplicate subform fields were removed. Each field can only be used once.'
  • Only one warning is displayed if two of the three duplicate fields are removed
  • No warning if two different fields are used in one subform field and the fields are still exist
  • No warning if two subform fields are using the same field

[muhme]

Thank you very much @krishnagandhicode for your contribution. Thanks to @brianteeman for your support. Thanks to @SRV-KILLER09 and @exlemor for testing.

[krishnagandhicode]

Thanks for the merge, and thanks to everyone who helped test.