Fix issue with duplicate column name

SwiftCSV/Array+duplicates.swift

@@ -0,0 +1,13 @@
+//
+//  Array+duplicates.swift
+//
+//
+//  Created by 胡逸飞 on 2024/4/26.
+//
+
+extension Array where Element: Hashable {
+    func duplicates() -> [Element] {
+        let counts = self.reduce(into: [:]) { counts, element in counts[element, default: 0] += 1 }
+        return counts.filter { $0.value > 1 }.map { $0.key }
+    }
+}

SwiftCSV/Parser.swift

@@ -126,6 +126,12 @@ enum Parser {
     static func enumerateAsDict(header: [String], content: String, delimiter: CSVDelimiter, rowLimit: Int? = nil, block: @escaping ([String : String]) -> ()) throws {
 
         let enumeratedHeader = header.enumerated()
+
+        // Check for duplicate column names
+        let duplicateColumns = header.duplicates()
+        if !duplicateColumns.isEmpty {
+            throw CSVParseError.duplicateColumns(columnNames: duplicateColumns)
+        }
 
         // Start after the header
         try enumerateAsArray(text: content, delimiter: delimiter, startAt: 1, rowLimit: rowLimit) { fields in

SwiftCSV/ParsingState.swift

@@ -9,8 +9,22 @@
 public enum CSVParseError: Error {
     case generic(message: String)
     case quotation(message: String)
+    case duplicateColumns(columnNames: [String])
+
+
+    public static func == (lhs: CSVParseError, rhs: CSVParseError) -> Bool {
+        switch (lhs, rhs) {
+        case (.generic(let message1), .generic(let message2)):
+            return message1 == message2
+        case (.quotation(let message1), .quotation(let message2)):
+            return message1 == message2
+        case (.duplicateColumns(let columns1), .duplicateColumns(let columns2)):
+            return columns1 == columns2
+        default:
+            return false
+        }
+    }
 }
-
 /// State machine of parsing CSV contents character by character.
 struct ParsingState {
 

SwiftCSVTests/DuplicateColumnNameHandlingTests.swift

@@ -0,0 +1,64 @@
+//
+//  DuplicateColumnNameHandlingTests.swift
+//
+//
+//  Created by 胡逸飞 on 2024/4/27.
+//
+
+import Foundation
+import XCTest
+@testable import SwiftCSV
+
+class DuplicateColumnNameHandlingTests: XCTestCase {
+
+    func testErrorOnDuplicateColumnNames() throws {
+        let csvString = """
+        id,name,age,name
+        1,John,23,John Doe
+        2,Jane,25,Jane Doe
+        """
+
+        XCTAssertThrowsError(try CSV<Named>(string: csvString)) { error in
+            switch error as? CSVParseError {
+            case .duplicateColumns(let columnNames):
+                XCTAssertEqual(["name"], columnNames)
+            default:
+                XCTFail("Expected CSVParseError.duplicateColumns")
+            }
+        }
+    }
+
+    func testNoDuplicateColumnNames() throws {
+        let csvString = """
+        id,name,age
+        1,John,23
+        2,Jane,25
+        """
+
+        let csvError = try CSV<Named>(string: csvString)
+        let csvRandom = try CSV<Named>(string: csvString)
+
+        XCTAssertEqual(csvError.header, ["id", "name", "age"])
+        XCTAssertEqual(csvRandom.header, ["id", "name", "age"])
+
+        XCTAssertEqual(csvError.rows.count, 2)
+        XCTAssertEqual(csvRandom.rows.count, 2)
+
+        XCTAssertEqual(csvError.rows[0]["id"], "1")
+        XCTAssertEqual(csvError.rows[0]["name"], "John")
+        XCTAssertEqual(csvError.rows[0]["age"], "23")
+
+        XCTAssertEqual(csvRandom.rows[0]["id"], "1")
+        XCTAssertEqual(csvRandom.rows[0]["name"], "John")
+        XCTAssertEqual(csvRandom.rows[0]["age"], "23")
+
+        XCTAssertEqual(csvError.rows[1]["id"], "2")
+        XCTAssertEqual(csvError.rows[1]["name"], "Jane")
+        XCTAssertEqual(csvError.rows[1]["age"], "25")
+
+        XCTAssertEqual(csvRandom.rows[1]["id"], "2")
+        XCTAssertEqual(csvRandom.rows[1]["name"], "Jane")
+        XCTAssertEqual(csvRandom.rows[1]["age"], "25")
+    }
+
+}