Deduplicate repeated target indices within a SwitchBuilder case to avoid duplicate delivery

[PratikDhanave (PratikDhanave)]

What

SwitchBuilder.collectTargets deduplicates the aggregated fan-out targets slice via targetIndexByID, but the per-call out slice appended an index for every binding passed with no membership check. Listing the same target twice within a single case or default — AddCase(pred, t1, t1) or WithDefault(t1, t1) — therefore produced indices [0, 0].

Nothing downstream deduplicates either: the assigner yields index 0 twice, selectedTargetIDs appends the same sink ID twice, resolveTargets returns the same executor twice, the type filter keeps both, and MapInto enqueues the envelope once per entry — so the matched message is delivered to that executor twice.

This change tracks a per-call seen set in collectTargets and emits each index at most once. It applies identically to AddCase and WithDefault since both route through collectTargets.

Why

Mirrors the .NET implementation, where a switch case's targets are backed by HashSet<int> so a repeated target collapses to a single delivery. This restores cross-SDK parity: a message routed through a switch should reach each distinct target exactly once regardless of how many times it was listed.

Testing

Added two black-box behavioral tests in workflow/builder_test.go using the existing recordingBinding harness:

• TestAddSwitch_DeduplicatesRepeatedTargetsWithinCase — AddCase(pred, target, target), asserts the target's handler fires exactly once.
• TestAddSwitch_DeduplicatesRepeatedDefaultTargets — WithDefault(def, def), asserts the default target fires exactly once.

Both fail before the fix (delivered twice) and pass after. go build ./..., go vet ./workflow/..., and go test ./workflow/... are green.

[Copilot]

Pull request overview

This PR fixes duplicate message delivery in SwitchBuilder when the same ExecutorBinding is listed multiple times within a single switch case or default branch. It aligns the Go SDK’s switch routing behavior with the .NET implementation by ensuring each distinct target receives a matched message at most once per case/default.

Changes:

• Deduplicate per-case/per-default target indices in SwitchBuilder.collectTargets using a per-call seen set.
• Add black-box tests to verify deduplication for repeated targets in a case and in the default branch.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
workflow/builder.go	Adds per-call deduplication in collectTargets to prevent duplicate deliveries when targets repeat within the same case/default.
workflow/builder_test.go	Adds behavioral tests ensuring repeated targets in a case/default only receive one delivery.

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[github-actions]

Parity Review — ✅ No Issues Found

Scope

This PR fixes a behavioral bug in the unexported SwitchBuilder.collectTargets helper in workflow/builder.go. The exported AddCase and WithDefault method signatures are unchanged; only the runtime behavior of repeated-target delivery is corrected.

Cross-SDK Parity

The fix directly mirrors the upstream .NET implementation:

• dotnet/src/Microsoft.Agents.AI.Workflows/SwitchBuilder.cs: case indices are stored in a HashSet<int> (per case in _caseMap) and _defaultIndicies is also a HashSet<int>. Inserting a duplicate index into a HashSet is a no-op, so the .NET implementation has always delivered each distinct target exactly once per matched case — even when the same ExecutorBinding is passed multiple times.
• Go (before this fix): collectTargets appended an index for every binding passed without a membership check, so AddCase(pred, t1, t1) produced [0, 0] and delivered the message twice.
• Go (after this fix): adds a seen set per call, emitting each index at most once — semantically equivalent to the .NET HashSet<int> approach.

No Python equivalent was found for SwitchBuilder in python/packages/, consistent with the known state of the Python SDK.

Labels

No exported Go API surface changed, so public-api-change is not warranted. The existing parity-approved label correctly reflects this review outcome.

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