properly report duplicate label names

compiler/src/prog8/compiler/astprocessing/AstIdentifiersChecker.kt

@@ -108,7 +108,7 @@ internal class AstIdentifiersChecker(private val errors: IErrorReporter,
             // the builtin functions can't be redefined
             errors.err("builtin function cannot be redefined", label.position)
         } else {
-            val existing = label.definingSubroutine?.getAllLabels(label.name) ?: emptyList()
+            val existing = (label.definingSubroutine ?: label.definingBlock).getAllLabels(label.name)
             for(el in existing) {
                 if(el === label || el.name != label.name)
                     continue

examples/test.p8

@@ -1,11 +1,25 @@
-%zeropage basicsafe
+%import textio
-
 main {
-    sub start() {
+	str myBar = "main.bar"
-        ubyte @shared xx
+
+foo_bar:
+
+	sub start() {
+	    txt.print(myBar)
+	    txt.print(&foo_bar)
+	    return
+
+quert:
+quert:
+quert:
+quert:
 
-        if xx==1 or xx==2 or xx==3 {
-            xx++
 	}
+
+        sub start() {
         }
+foo_bar:
+foo_bar:
+foo_bar:
+foo_bar:
 }