Simplify and refactor deduplication into standalone method

Sources/SourceKitLSP/TestDiscovery.swift

@@ -273,6 +273,7 @@ extension SourceKitLSPServer {
       .flatMap { $0 }
       .sorted { $0.testItem.location < $1.testItem.location }
       .mergingTestsInExtensions()
+      .deduplicatingIds()
   }
 
   func documentTests(
@@ -283,6 +284,7 @@ extension SourceKitLSPServer {
     return try await documentTestsWithoutMergingExtensions(req, workspace: workspace, languageService: languageService)
       .prefixTestsWithModuleName(workspace: workspace)
       .mergingTestsInExtensions()
+      .deduplicatingIds()
   }
 
   private func documentTestsWithoutMergingExtensions(
@@ -441,16 +443,20 @@ fileprivate extension Array<AnnotatedTestItem> {
         // as the root item.
         if rootItem.isExtension && !item.isExtension {
           var newItem = item
-          newItem.testItem.children = (newItem.testItem.children + rootItem.testItem.children).deduplicatingIds()
+          newItem.testItem.children += rootItem.testItem.children
           rootItem = newItem
-        } else if rootItem.testItem.children.isEmpty && item.testItem.children.isEmpty {
-          itemDict[item.testItem.ambiguousTestDifferentiator] = item
-          continue
         } else {
-          rootItem.testItem.children = (rootItem.testItem.children + item.testItem.children).deduplicatingIds()
+          rootItem.testItem.children += item.testItem.children
         }
 
-        itemDict[id] = rootItem
+        // If this item shares an ID with a sibling and both are leaf
+        // test items, store it by its disambiguated id to ensure we
+        // don't overwrite the existing element.
+        if rootItem.testItem.children.isEmpty && item.testItem.children.isEmpty {
+          itemDict[item.testItem.ambiguousTestDifferentiator] = item
+        } else {
+          itemDict[id] = rootItem
+        }
       } else {
         itemDict[id] = item
       }
@@ -487,7 +493,7 @@ fileprivate extension Array<AnnotatedTestItem> {
         .mergingTestsInExtensions()
       return newItem
     }
-    return result.deduplicatingIds()
+    return result
   }
 
   func prefixTestsWithModuleName(workspace: Workspace) async -> Self {
@@ -520,32 +526,18 @@ fileprivate extension Array<TestItem> {
   /// by appending `/filename:filename:lineNumber`.
   func deduplicatingIds() -> [TestItem] {
     var idCounts: [String: Int] = [:]
-    var result: [TestItem] = []
-    var hasDuplicates = false
-    result.reserveCapacity(self.count)
-
     for element in self where element.children.isEmpty {
       idCounts[element.id, default: 0] += 1
-      if idCounts[element.id, default: 0] > 1 {
-        hasDuplicates = true
-      }
     }
 
-    if !hasDuplicates {
-      return self
-    }
-
-    for element in self {
-      if idCounts[element.id, default: 0] > 1 {
-        var newItem = element
+    return self.map {
+      var newItem = $0
+      newItem.children = newItem.children.deduplicatingIds()
+      if idCounts[newItem.id, default: 0] > 1 {
         newItem.id = newItem.ambiguousTestDifferentiator
-        result.append(newItem)
-      } else {
-        result.append(element)
       }
+      return newItem
     }
-
-    return result
   }
 }
 

Tests/SourceKitLSPTests/DocumentTestDiscoveryTests.swift

@@ -455,6 +455,53 @@ final class DocumentTestDiscoveryTests: XCTestCase {
     )
   }
 
+  func testSwiftTestingTestsWithDuplicateFunctionIdentifiersInSuite() async throws {
+    let testClient = try await TestSourceKitLSPClient()
+    let uri = DocumentURI(for: .swift)
+
+    let positions = testClient.openDocument(
+      """
+      import Testing
+
+      1️⃣struct MySuite {
+        3️⃣@Test(arguments: [1, 2, 3])
+        func foo(_ x: Int) {}4️⃣
+        5️⃣@Test(arguments: ["a", "b", "c"])
+        func foo(_ x: String) {}6️⃣
+      }2️⃣
+      """,
+      uri: uri
+    )
+
+    let filename = uri.fileURL?.lastPathComponent ?? ""
+    let tests = try await testClient.send(DocumentTestsRequest(textDocument: TextDocumentIdentifier(uri)))
+    XCTAssertEqual(
+      tests,
+      [
+        TestItem(
+          id: "MySuite",
+          label: "MySuite",
+          style: TestStyle.swiftTesting,
+          location: Location(uri: uri, range: positions["1️⃣"]..<positions["2️⃣"]),
+          children: [
+            TestItem(
+              id: "MySuite/foo(_:)/\(filename):\(positions["3️⃣"].line + 1):\(positions["3️⃣"].utf16index + 2)",
+              label: "foo(_:)",
+              style: TestStyle.swiftTesting,
+              location: Location(uri: uri, range: positions["3️⃣"]..<positions["4️⃣"])
+            ),
+            TestItem(
+              id: "MySuite/foo(_:)/\(filename):\(positions["5️⃣"].line + 1):\(positions["5️⃣"].utf16index + 2)",
+              label: "foo(_:)",
+              style: TestStyle.swiftTesting,
+              location: Location(uri: uri, range: positions["5️⃣"]..<positions["6️⃣"])
+            ),
+          ]
+        )
+      ]
+    )
+  }
+
   func testSwiftTestingTestsWithDuplicateFunctionIdentifiersInExtension() async throws {
     let testClient = try await TestSourceKitLSPClient()
     let uri = DocumentURI(for: .swift)