Report warning for non-boolean CLI argument passed multiple times

Only if its values are different, see the test

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/CommonCompilerArguments.java

@@ -20,7 +20,9 @@ import com.intellij.util.SmartList;
 import org.jetbrains.annotations.NotNull;
 
 import java.io.Serializable;
+import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Map;
 
 public abstract class CommonCompilerArguments implements Serializable {
     public static final long serialVersionUID = 0L;
@@ -105,6 +107,10 @@ public abstract class CommonCompilerArguments implements Serializable {
     // Names of extra (-X...) arguments which have been passed in an obsolete form ("-Xaaa bbb", instead of "-Xaaa=bbb")
     public List<String> extraArgumentsPassedInObsoleteForm = new SmartList<>();
 
+    // Non-boolean arguments which have been passed multiple times, possibly with different values.
+    // The key in the map is the name of the argument, the value is the last passed value.
+    public Map<String, String> duplicateArguments = new LinkedHashMap<>();
+
     @NotNull
     public static CommonCompilerArguments createDefaultInstance() {
         return new DummyImpl();

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/parseCommandLineArguments.kt

@@ -30,6 +30,7 @@ val Argument.isAdvanced: Boolean
 
 private val ADVANCED_ARGUMENT_PREFIX = "-X"
 
+// Parses arguments in the passed [result] object, or throws an [IllegalArgumentException] with the message to be displayed to the user
 fun <A : CommonCompilerArguments> parseCommandLineArguments(args: Array<String>, result: A) {
     data class ArgumentField(val field: Field, val argument: Argument)
 
@@ -38,6 +39,8 @@ fun <A : CommonCompilerArguments> parseCommandLineArguments(args: Array<String>,
         if (argument != null) ArgumentField(field, argument) else null
     }
 
+    val visitedArgs = mutableSetOf<String>()
+
     var i = 0
     while (i < args.size) {
         val arg = args[i++]
@@ -74,6 +77,10 @@ fun <A : CommonCompilerArguments> parseCommandLineArguments(args: Array<String>,
             }
         }
 
+        if (!field.type.isArray && !visitedArgs.add(argument.value) && value is String && field.get(result) != value) {
+            result.duplicateArguments.put(argument.value, value)
+        }
+
         updateField(field, result, value)
     }
 }

compiler/cli/src/org/jetbrains/kotlin/cli/common/CLICompiler.java

@@ -136,7 +136,7 @@ public abstract class CLICompiler<A extends CommonCompilerArguments> {
             messageCollector = new FilteringMessageCollector(messageCollector, Predicate.isEqual(WARNING));
         }
 
-        reportUnknownAndObsoleteExtraFlags(messageCollector, arguments);
+        reportArgumentParseProblems(messageCollector, arguments);
 
         GroupingMessageCollector groupingCollector = new GroupingMessageCollector(messageCollector);
 
@@ -311,7 +311,7 @@ public abstract class CLICompiler<A extends CommonCompilerArguments> {
             @NotNull CompilerConfiguration configuration, @NotNull A arguments, @NotNull Services services
     );
 
-    private void reportUnknownAndObsoleteExtraFlags(@NotNull MessageCollector collector, @NotNull A arguments) {
+    private void reportArgumentParseProblems(@NotNull MessageCollector collector, @NotNull A arguments) {
         for (String flag : arguments.unknownExtraFlags) {
             collector.report(STRONG_WARNING, "Flag is not supported by this version of the compiler: " + flag, null);
         }
@@ -319,6 +319,10 @@ public abstract class CLICompiler<A extends CommonCompilerArguments> {
             collector.report(STRONG_WARNING, "Advanced option value is passed in an obsolete form. Please use the '=' character " +
                                              "to specify the value: " + argument + "=...", null);
         }
+        for (Map.Entry<String, String> argument : arguments.duplicateArguments.entrySet()) {
+            collector.report(STRONG_WARNING, "Argument " + argument.getKey() + " is passed multiple times. " +
+                                             "Only the last value will be used: " + argument.getValue(), null);
+        }
     }
 
     @NotNull