added check for duplicate label definitions

compiler/src/prog8/ast/AstToplevel.kt

@@ -158,6 +158,30 @@ interface INameScope {
         if(!statements.remove(stmt))
             throw FatalAstException("stmt to remove wasn't found in scope")
     }
+
+    fun getAllLabels(label: String): List<Label> {
+        val result = mutableListOf<Label>()
+
+        fun find(scope: INameScope) {
+            scope.statements.forEach {
+                when(it) {
+                    is Label -> result.add(it)
+                    is INameScope -> find(it)
+                    is IfStatement -> {
+                        find(it.truepart)
+                        find(it.elsepart)
+                    }
+                    is RepeatLoop -> find(it.body)
+                    is ForeverLoop -> find(it.body)
+                    is WhileLoop -> find(it.body)
+                    is WhenStatement -> it.choices.forEach { choice->find(choice.statements) }
+                }
+            }
+        }
+
+        find(this)
+        return result
+    }
 }
 
 interface IAssignable {

compiler/src/prog8/ast/processing/AstIdentifiersChecker.kt

@@ -165,9 +165,15 @@ internal class AstIdentifiersChecker(private val program: Program,
             // the builtin functions can't be redefined
             errors.err("builtin function cannot be redefined", label.position)
         } else {
-            val existing = program.namespace.lookup(listOf(label.name), label)
-            if (existing != null && existing !== label)
-                nameError(label.name, label.position, existing)
+            val existing = label.definingSubroutine()?.getAllLabels(label.name) ?: emptyList()
+            for(el in existing) {
+                if(el === label || el.name != label.name)
+                    continue
+                else {
+                    nameError(label.name, label.position, el)
+                    break
+                }
+            }
         }
         return super.visit(label)
     }

compiler/src/prog8/optimizer/StatementOptimizer.kt

@@ -15,7 +15,7 @@ import kotlin.math.floor
 
 
 /*
-    TODO: remove unreachable code?
+    TODO: remove unreachable code after return and exit()
     TODO: proper inlining of tiny subroutines (at first, restrict to subs without parameters and variables in them, and build it up from there: correctly renaming/relocating all variables in them and refs to those as well)
 */