adding duplicate modifier test

Author: swhitty

SwiftDraw/CommandLine.Arguments.swift

@@ -50,7 +50,7 @@ extension CommandLine {
         var args = args
         var modifiers = [Modifier: String]()
         while let pair = args.takePair() {
-            if let modifier = Modifier.parse(from: pair.0) {
+            if let modifier = Modifier.parse(from: pair.0), modifiers.keys.contains(modifier) == false  {
                 modifiers[modifier] = pair.1
             } else {
                 throw Error.invalid

SwiftDrawTests/CommandLine.ArgumentsTests.swift

@@ -44,6 +44,11 @@ final class CommandLineArgumentsTests: XCTestCase {
         XCTAssertThrowsError(try CommandLine.parseModifiers(from: ["--format", "png", "--output"]))
     }
 
+    func testParseModifiersThrowsForDuplicateModifiers() {
+        XCTAssertThrowsError(try CommandLine.parseModifiers(from: ["--format", "png", "--format", "jpg"]))
+        XCTAssertThrowsError(try CommandLine.parseModifiers(from:  ["--format", "png", "--output", "more", "--output", "evenmore"]))
+    }
+
     func testParseModifiersThrowsForUnknownModifiers() {
         XCTAssertThrowsError(try CommandLine.parseModifiers(from: ["--unknown", "png"]))
         XCTAssertThrowsError(try CommandLine.parseModifiers(from: ["--format", "png", "--unknown", "more"]))