[Muhammad Abdullah Butt (1AbdullahButt)]

Motivation and Context

Fixes #14786.

When two streaming chunks are added, StreamingContentMixin._merge_items_lists merged each incoming item into every compatible item in the list. For example, a first chunk with two text parts plus a second chunk with "tail" gave ['Part one. tail', 'Part two. tail'], so the text was duplicated.

Description

The inner loop set added = True after a successful item + other_item but kept iterating. It now breaks at the first successful merge, so each incoming item is merged into exactly one item, as the existing tests already assume for the single-item case.

Added test_scmc_add_merges_item_into_one_matching_item_only to tests/unit/contents/test_streaming_chat_message_content.py. It fails before the change and passes after.

Tested locally (Python 3.12, Windows):

• the new test fails without the fix and passes with it
• test_streaming_chat_message_content.py, test_function_call_content.py and test_streaming_annotation_content.py: 82 passed, 0 failed. I ran them with --noconftest, so the 9 tests that need conftest fixtures (function_call) were not run.
• ruff check and ruff format --check on both files pass

Claude Code wrote this change.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

No active review issues are identified for the change to prevent duplicate text during streaming-content merging.

No open issues 🎉

What changed in this PR

Fixes #14786 by ensuring Python streaming-content merging adds each incoming item to only the first compatible item, preventing duplicated text across matching items.

Changes:

• Stops the inner merge loop after the first successful addition.
• Adds a regression test confirming that only the first of two compatible text items receives the incoming text.

File	Description
python/​tests/​unit/​contents/​test_streaming_chat_message_content.py	Adds regression coverage for merging into only one matching item.
python/​semantic_kernel/​contents/​streaming_content_mixin.py	Breaks the loop after a successful merge to prevent duplicate additions.

_{Review effort: Balanced}