SwiftFixIt: Skip duplicate insertion fix-its

There are various scenarios — most notably involving protocol
compositions — where multiple different sites are diagnosed with the
same insertion fix-it. For example, compiling the following code with
`-enable-upcoming-feature InferIsolatedConformances:migrate` generates 2
warnings with equal fix-its for prepending `nonisolated` to the
composition:

```swift
protocol P {}
protocol Q {}

@mainactor struct S: P & Q {}
```

Applying both fix-its will produce invalid code:
`@MainActor struct S: nonisolated nonisolated P & Q {}`.

Author: AnthonyLatsis

Fixtures/SwiftMigrate/ExistentialAnyMigration/Sources/Fixed/Test.swift

@@ -1,5 +1,7 @@
 protocol P {
 }
+protocol Q {
+}
 
 func test1(_: any P) {
 }
@@ -14,3 +16,6 @@ func test3() {
 func test4() {
     var x = 42
 }
+
+func test5(_: any P & Q) {
+}

Fixtures/SwiftMigrate/ExistentialAnyMigration/Sources/Test.swift

@@ -1,5 +1,7 @@
 protocol P {
 }
+protocol Q {
+}
 
 func test1(_: P) {
 }
@@ -14,3 +16,6 @@ func test3() {
 func test4() {
     var x = 42
 }
+
+func test5(_: P & Q) {
+}

Fixtures/SwiftMigrate/InferIsolatedConformancesMigration/Sources/Fixed/Test.swift

@@ -6,3 +6,9 @@ class C: nonisolated Equatable {
     lhs.name == rhs.name
   }
 }
+
+protocol P {}
+protocol Q {}
+
+@MainActor
+struct S: nonisolated P & Q {}

Fixtures/SwiftMigrate/InferIsolatedConformancesMigration/Sources/Test.swift

@@ -6,3 +6,9 @@ class C: Equatable {
     lhs.name == rhs.name
   }
 }
+
+protocol P {}
+protocol Q {}
+
+@MainActor
+struct S: P & Q {}

Sources/SwiftFixIt/SwiftFixIt.swift

@@ -301,7 +301,11 @@ extension SwiftFixIt {
                 }
             }
 
-            let result = SwiftIDEUtils.FixItApplier.apply(edits: consume edits, to: sourceFile.syntax)
+            let result = SwiftIDEUtils.FixItApplier.apply(
+                edits: consume edits,
+                to: sourceFile.syntax,
+                allowDuplicateInsertions: false
+            )
 
             try self.fileSystem.writeFileContents(sourceFile.path, string: consume result)
         }

Tests/CommandsTests/PackageCommandTests.swift

@@ -2151,9 +2151,9 @@ class PackageCommandTestCase: CommandsBuildProviderTestCase {
             }
         }
 
-        try await doMigration(featureName: "ExistentialAny", expectedSummary: "Applied 3 fix-its in 1 file")
+        try await doMigration(featureName: "ExistentialAny", expectedSummary: "Applied 5 fix-its in 1 file")
         try await doMigration(featureName: "StrictMemorySafety", expectedSummary: "Applied 1 fix-it in 1 file")
-        try await doMigration(featureName: "InferIsolatedConformances", expectedSummary: "Applied 1 fix-it in 1 file")
+        try await doMigration(featureName: "InferIsolatedConformances", expectedSummary: "Applied 3 fix-its in 1 file")
     }
 
     func testBuildToolPlugin() async throws {

Tests/SwiftFixItTests/FilteringTests.swift

@@ -412,19 +412,12 @@ struct FilteringTests {
         }
     }
 
-    @Test
     func testDuplicateInsertionFixIts() throws {
-        withKnownIssue("FIXME: Filter out duplicate insertion fix-its") {
-            try self._testDuplicateInsertionFixIts()
-        }
-    }
-
-    func _testDuplicateInsertionFixIts() throws {
         try testAPI1File { path in
             .init(
                 edits: .init(input: "var x = 1", result: "@W var yx = 21"),
                 summary: .init(
-                    // 4 because skipped by SwiftIDEUtils.FixItApplier, not SwiftFixIt.
+                    // 6 because skipped by SwiftIDEUtils.FixItApplier, not SwiftFixIt.
                     numberOfFixItsApplied: 6,
                     numberOfFilesChanged: 1
                 ),
@@ -470,7 +463,7 @@ struct FilteringTests {
                         text: "error3_fixit1",
                         location: .init(path: path, line: 1, column: 3),
                         fixIts: [
-                            // FIXME: Should be skipped.
+                            // Skipped, duplicate insertion.
                             .init(
                                 start: .init(path: path, line: 1, column: 1),
                                 end: .init(path: path, line: 1, column: 1),
@@ -500,7 +493,7 @@ struct FilteringTests {
                                 text: "error5_note1",
                                 location: .init(path: path, line: 1, column: 9),
                                 fixIts: [
-                                    // FIXME: Should be skipped.
+                                    // Skipped, duplicate insertion.
                                     .init(
                                         start: .init(path: path, line: 1, column: 9),
                                         end: .init(path: path, line: 1, column: 9),
@@ -519,7 +512,7 @@ struct FilteringTests {
                                 text: "error6_note1",
                                 location: .init(path: path, line: 1, column: 5),
                                 fixIts: [
-                                    // FIXME: Should be skipped.
+                                    // Skipped, duplicate insertion.
                                     .init(
                                         start: .init(path: path, line: 1, column: 5),
                                         end: .init(path: path, line: 1, column: 5),