fix: reject duplicate if-block conditions within the same list key

[Tobias-Fischer]

Summary

• Two "- if:" blocks for the same condition under one list key (e.g. two separate "if: win" blocks in packages_select_by_deps) are never wrong on their own, but they're a standing foot-gun downstream: a future addition can land in the "wrong" one by accident, nothing merges the two, and the same package can end up skipped from one angle and not the other depending on which block someone edits later.
• This is distinct from the adjacent-blocks bug fixed in fix: close if-block before starting a new one in sort_vinca_lists #152 — these duplicates aren't adjacent, so that fix's "close the block on a new - if: line" logic never sees them as related at all.
• Raises DuplicateConditionError (exit 1, clean message naming the list key and condition) rather than silently merging the blocks — merging correctly needs a human to reconcile any per-item comments, since sorting the merged then: list can separate a standalone comment from the item it was meant to describe.

Motivation

Found while auditing RoboStack/ros-humble, ros-jazzy, and ros-rolling's vinca.yaml files for exactly this pattern. Humble alone had accumulated 4 separate duplicate-condition groups across ~19 blocks (mostly if: not win and if: linux) from being extended incrementally across many sessions — consolidating them by hand also surfaced live cases where a per-item comment had already drifted onto the wrong package from a previous sort, which is what motivated making the check hard-fail rather than silently reformat.

Test plan

• pytest vinca/ — all 236 tests pass (7 in test_sort_vinca_lists.py, including 3 new ones for this change: duplicate-in-same-list raises, duplicate-across-different-list-keys is fine, different-conditions-in-same-list is fine)
• ruff check / ruff format --check clean
• Ran the updated checker against the already-consolidated humble/jazzy/rolling vinca.yaml files — all pass clean with no duplicates
• Verified against a synthetic non-adjacent duplicate (- if: win ... other content ... - if: win) that it's correctly caught with a clear message

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