diff --git a/il65/src/il65/ast/AST.kt b/il65/src/il65/ast/AST.kt
index 60d0631a1..c053853b6 100644
--- a/il65/src/il65/ast/AST.kt
+++ b/il65/src/il65/ast/AST.kt
@@ -48,6 +48,7 @@ data class Position(val file: String, val line: Int, val startCol: Int, val endC
 
 
 interface IAstProcessor {
+    // override the ones you want to act upon
     fun process(module: Module) {
     }
     fun process(expr: PrefixExpression): IExpression {
diff --git a/il65/src/il65/ast/AstChecker.kt b/il65/src/il65/ast/AstChecker.kt
index 2f4c792e6..f0da99d83 100644
--- a/il65/src/il65/ast/AstChecker.kt
+++ b/il65/src/il65/ast/AstChecker.kt
@@ -1,7 +1,6 @@
 package il65.ast
 
 import il65.ParsingFailedError
-import javax.xml.crypto.Data
 
 
 fun Module.checkValid() {
@@ -50,6 +49,18 @@ class AstChecker : IAstProcessor {
             blockNames[block.name] = block.position
         }
         block.statements.forEach { it.process(this) }
+
+        // check if labels are unique
+        val labels = block.statements.filter { it is Label }.map { it as Label }
+        val labelnames = mutableMapOf<String, Position?>()
+        labels.forEach {
+            val existing = labelnames[it.name]
+            if(existing!=null) {
+                checkResult.add(SyntaxError("label name conflict, first defined on line ${existing.line}", it.position))
+            } else {
+                labelnames[it.name] = it.position
+            }
+        }
         return block
     }
 
@@ -72,6 +83,18 @@ class AstChecker : IAstProcessor {
 
         subroutine.statements.forEach { it.process(this) }
 
+        // check if labels are unique
+        val labels = subroutine.statements.filter { it is Label }.map { it as Label }
+        val labelnames = mutableMapOf<String, Position?>()
+        labels.forEach {
+            val existing = labelnames[it.name]
+            if(existing!=null) {
+                checkResult.add(SyntaxError("label name conflict, first defined on line ${existing.line}", it.position))
+            } else {
+                labelnames[it.name] = it.position
+            }
+        }
+
         // subroutine must contain at least one 'return' or 'goto'
         // (or if it has an asm block, that must contain a 'rts' or 'jmp')
         if(subroutine.statements.count { it is Return || it is Jump } == 0) {