Reject duplicate headers in dictionary views

SwiftCSV/Parser.swift

@@ -28,6 +28,8 @@ extension CSV {
         try Parser.enumerateAsArray(text: self.text, delimiter: self.delimiter, startAt: startAt, rowLimit: rowLimit, rowCallback: rowCallback)
     }
 
+    /// Enumerate each row as a dictionary keyed by its header names.
+    /// - Throws: `CSVParseError.duplicateHeader` if headers repeat, or an error thrown by `block`.
     public func enumerateAsDict(_ block: @escaping ([String : String]) throws -> ()) throws {
 
         try Parser.enumerateAsDict(header: self.header, content: self.text, delimiter: self.delimiter, block: block)
@@ -125,8 +127,15 @@ enum Parser {
         }
     }
 
+    /// Parse rows as dictionaries. Duplicate header names are rejected because a dictionary cannot
+    /// preserve both columns.
     static func enumerateAsDict(header: [String], content: String, delimiter: CSVDelimiter, rowLimit: Int? = nil, block: @escaping ([String : String]) throws -> ()) throws {
 
+        var seenHeaders = Set<String>()
+        for name in header where !seenHeaders.insert(name).inserted {
+            throw CSVParseError.duplicateHeader(name: name)
+        }
+
         let enumeratedHeader = header.enumerated()
 
         // Start after the header

SwiftCSV/ParsingState.swift

@@ -9,6 +9,8 @@
 public enum CSVParseError: Error {
     case generic(message: String)
     case quotation(message: String)
+    /// A duplicate header cannot be represented without losing a column in dictionary-based access.
+    case duplicateHeader(name: String)
 }
 
 /// State machine of parsing CSV contents character by character.

SwiftCSVTests/DuplicateHeaderTests.swift

@@ -0,0 +1,37 @@
+import XCTest
+@testable import SwiftCSV
+
+final class DuplicateHeaderTests: XCTestCase {
+    private let text = """
+    en,comment,de,en,en-GB,fr,es,it
+    ,,,,,,,
+    Efficiency,,,,,,,
+    %@,,,,,,,
+    """
+
+    func testNamedViewRejectsDuplicateHeaders() {
+        XCTAssertThrowsError(try CSV<Named>(string: text)) { error in
+            guard case CSVParseError.duplicateHeader(name: "en") = error else {
+                return XCTFail("Expected a duplicate-header error, got \(error)")
+            }
+        }
+    }
+
+    func testEnumeratedViewPreservesDuplicateHeaderColumns() throws {
+        let csv = try CSV<Enumerated>(string: text)
+
+        XCTAssertEqual(csv.header, ["en", "comment", "de", "en", "en-GB", "fr", "es", "it"])
+        XCTAssertEqual(csv.rows[1], ["Efficiency", "", "", "", "", "", "", ""])
+        XCTAssertEqual(csv.columns?.filter { $0.header == "en" }.count, 2)
+    }
+
+    func testDictionaryEnumerationRejectsDuplicateHeaders() throws {
+        let csv = try CSV<Enumerated>(string: text)
+
+        XCTAssertThrowsError(try csv.enumerateAsDict { _ in }) { error in
+            guard case CSVParseError.duplicateHeader(name: "en") = error else {
+                return XCTFail("Expected a duplicate-header error, got \(error)")
+            }
+        }
+    }
+}