diff --git a/tool/src/org/antlr/v4/semantics/SemanticPipeline.java b/tool/src/org/antlr/v4/semantics/SemanticPipeline.java
index 89adf24df..da87b3b64 100644
--- a/tool/src/org/antlr/v4/semantics/SemanticPipeline.java
+++ b/tool/src/org/antlr/v4/semantics/SemanticPipeline.java
@@ -221,16 +221,13 @@ public class SemanticPipeline {
 	{
 		//Grammar G = g.getOutermostGrammar(); // put in root, even if imported
 
-		// DEFINE tokens { X='x'; } ALIASES
+		// create token types for tokens { A, B, C } ALIASES
 		for (GrammarAST alias : tokensDefs) {
-			if ( alias.getType()== ANTLRParser.ASSIGN ) {
-				String name = alias.getChild(0).getText();
-				String lit = alias.getChild(1).getText();
-				g.defineTokenAlias(name, lit);
-			}
-			else {
-				g.defineTokenName(alias.getText());
+			if (g.getTokenType(alias.getText()) != Token.INVALID_TYPE) {
+				g.tool.errMgr.grammarError(ErrorType.TOKEN_NAME_REASSIGNMENT, g.fileName, alias.token, alias.getText());
 			}
+
+			g.defineTokenName(alias.getText());
 		}
 
 		// DEFINE TOKEN TYPES FOR TOKEN REFS LIKE ID, INT
diff --git a/tool/src/org/antlr/v4/tool/ErrorType.java b/tool/src/org/antlr/v4/tool/ErrorType.java
index 51a969544..b34cd04f6 100644
--- a/tool/src/org/antlr/v4/tool/ErrorType.java
+++ b/tool/src/org/antlr/v4/tool/ErrorType.java
@@ -126,7 +126,7 @@ public enum ErrorType {
 	NO_SUCH_GRAMMAR_SCOPE(105, "reference to undefined grammar in rule reference: <arg>.<arg2>", ErrorSeverity.ERROR),
 	NO_SUCH_RULE_IN_SCOPE(106, "rule <arg2> is not defined in grammar <arg>", ErrorSeverity.ERROR),
 //	TOKEN_STRING_REASSIGNMENT(107, "cannot alias <arg> in tokens {}; string already assigned to <arg2>", ErrorSeverity.ERROR),
-//	TOKEN_NAME_REASSIGNMENT(108, "cannot redefine <arg>; token name already <if(arg2)>assigned to <arg2><else>defined<endif>", ErrorSeverity.ERROR),
+	TOKEN_NAME_REASSIGNMENT(108, "token name <arg> is already defined", ErrorSeverity.WARNING),
 	//TOKEN_VOCAB_IN_DELEGATE(, "tokenVocab option ignored in imported grammar <arg>", ErrorSeverity.ERROR),
 	OPTIONS_IN_DELEGATE(109, "options ignored in imported grammar <arg>", ErrorSeverity.WARNING),
 //	TOKEN_ALIAS_IN_DELEGATE(, "can't assign string to token name <arg> to string in imported grammar <arg2>", ErrorSeverity.ERROR),