Fix intersect and subtract

Author: Pouya Yarandi

Sources/SwiftProtobuf/Google_Protobuf_FieldMask+Extensions.swift

@@ -282,7 +282,10 @@ extension Google_Protobuf_FieldMask {
   ) -> Google_Protobuf_FieldMask {
     let set = mask.pathsSet
     var _paths: [String] = []
-    for path in paths where set.contains(path) {
+    var _buffer = Set<String>()
+    for path in paths where set.contains(path)
+    && !_buffer.contains(path) {
+      _buffer.insert(path)
       _paths.append(path)
     }
     return .with { mask in
@@ -300,7 +303,10 @@ extension Google_Protobuf_FieldMask {
   ) -> Google_Protobuf_FieldMask {
     let set = mask.pathsSet
     var _paths: [String] = []
-    for path in paths where !set.contains(path) {
+    var _buffer = Set<String>()
+    for path in paths where !set.contains(path)
+    && !_buffer.contains(path) {
+      _buffer.insert(path)
       _paths.append(path)
     }
     return .with { mask in

Tests/SwiftProtobufTests/Test_FieldMask.swift

@@ -279,6 +279,10 @@ final class Test_FieldMask: XCTestCase, PBTestHelpers {
         let m7 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
         let m8 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
         XCTAssertEqual(m7.union(m8).paths, ["a", "b"])
+
+        let m9 = Google_Protobuf_FieldMask(protoPaths: ["a", "a"])
+        let m10 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
+        XCTAssertEqual(m9.union(m10).paths, ["a", "b"])
     }
 
     // Checks `intersect` func of fieldMask.
@@ -298,6 +302,10 @@ final class Test_FieldMask: XCTestCase, PBTestHelpers {
         let m7 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
         let m8 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
         XCTAssertEqual(m7.intersect(m8).paths, ["a", "b"])
+
+        let m9 = Google_Protobuf_FieldMask(protoPaths: ["a", "a"])
+        let m10 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
+        XCTAssertEqual(m9.intersect(m10).paths, ["a"])
     }
 
     // Checks `substract` func of fieldMask.
@@ -317,6 +325,10 @@ final class Test_FieldMask: XCTestCase, PBTestHelpers {
         let m7 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
         let m8 = Google_Protobuf_FieldMask(protoPaths: ["a", "b"])
         XCTAssertEqual(m7.subtract(m8).paths, [])
+
+        let m9 = Google_Protobuf_FieldMask(protoPaths: ["a", "a"])
+        let m10 = Google_Protobuf_FieldMask(protoPaths: ["b"])
+        XCTAssertEqual(m9.subtract(m10).paths, ["a"])
     }
 
 }