diff --git a/compiler/src/prog8/compiler/astprocessing/VariousCleanups.kt b/compiler/src/prog8/compiler/astprocessing/VariousCleanups.kt
index 86cea5abb..48d52d8b2 100644
--- a/compiler/src/prog8/compiler/astprocessing/VariousCleanups.kt
+++ b/compiler/src/prog8/compiler/astprocessing/VariousCleanups.kt
@@ -62,10 +62,18 @@ internal class VariousCleanups(val program: Program, val errors: IErrorReporter,
     }
 
     override fun after(assignment: Assignment, parent: Node): Iterable<IAstModification> {
-        val nextAssign = assignment.nextSibling() as? Assignment
-        if(nextAssign!=null && nextAssign.target.isSameAs(assignment.target, program)) {
-            if(!nextAssign.isAugmentable && nextAssign.value isSameAs assignment.value && assignment.value !is IFunctionCall)    // don't remove function calls even when they're duplicates
-                return listOf(IAstModification.Remove(assignment, parent as IStatementContainer))
+        // remove duplicated assignments, but not if it's a memory mapped IO register
+        val isIO = try {
+            assignment.target.isIOAddress(options.compTarget.machine)
+        } catch (_: FatalAstException) {
+            false
+        }
+        if(!isIO) {
+            val nextAssign = assignment.nextSibling() as? Assignment
+            if (nextAssign != null && nextAssign.target.isSameAs(assignment.target, program)) {
+                if (!nextAssign.isAugmentable && nextAssign.value isSameAs assignment.value && assignment.value !is IFunctionCall)    // don't remove function calls even when they're duplicates
+                    return listOf(IAstModification.Remove(assignment, parent as IStatementContainer))
+            }
         }
 
         return noModifications
diff --git a/compiler/test/TestOptimization.kt b/compiler/test/TestOptimization.kt
index 633d96691..56501d232 100644
--- a/compiler/test/TestOptimization.kt
+++ b/compiler/test/TestOptimization.kt
@@ -15,6 +15,7 @@ import prog8.ast.statements.*
 import prog8.code.core.DataType
 import prog8.code.core.Position
 import prog8.code.target.C64Target
+import prog8.code.target.Cx16Target
 import prog8.compiler.printProgram
 import prog8tests.helpers.*
 
@@ -736,4 +737,60 @@ class TestOptimization: FunSpec({
         val stmts = result.program.entrypoint.statements
         stmts.size shouldBe 3
     }
+
+    test("repeated assignments to IO register should remain") {
+        val srcX16="""
+main {
+    sub start() {
+        ubyte @shared xx
+        xx = 42
+        xx = 42  ; removed
+        xx = 42  ; removed
+        cx16.VERA_DATA0 = 0
+        cx16.VERA_DATA0 = 0
+        cx16.VERA_DATA0 = 0
+        @($9fff) = 0
+        @($9fff) = 0
+        @($9fff) = 0
+        return
+    }
+}"""
+        var result = compileText(Cx16Target(), true, srcX16, writeAssembly = true)!!
+        var statements = result.program.entrypoint.statements
+        statements.size shouldBe 9
+        (statements[1] as Assignment).target.identifier!!.nameInSource shouldBe listOf("xx")
+        (statements[2] as Assignment).target.identifier!!.nameInSource shouldBe listOf("cx16", "VERA_DATA0")
+        (statements[3] as Assignment).target.identifier!!.nameInSource shouldBe listOf("cx16", "VERA_DATA0")
+        (statements[4] as Assignment).target.identifier!!.nameInSource shouldBe listOf("cx16", "VERA_DATA0")
+        (statements[5] as Assignment).target.memoryAddress!!.addressExpression.constValue(result.program)!!.number shouldBe 0x9fff
+        (statements[6] as Assignment).target.memoryAddress!!.addressExpression.constValue(result.program)!!.number shouldBe 0x9fff
+        (statements[7] as Assignment).target.memoryAddress!!.addressExpression.constValue(result.program)!!.number shouldBe 0x9fff
+
+        val srcC64="""
+main {
+    sub start() {
+        ubyte @shared xx
+        xx = 42
+        xx = 42  ;removed
+        xx = 42  ;removed
+        c64.EXTCOL = 0
+        c64.EXTCOL = 0
+        c64.EXTCOL = 0
+        @(53281) = 0
+        @(53281) = 0
+        @(53281) = 0
+        return
+    }
+}"""
+        result = compileText(C64Target(), true, srcC64, writeAssembly = true)!!
+        statements = result.program.entrypoint.statements
+        statements.size shouldBe 9
+        (statements[1] as Assignment).target.identifier!!.nameInSource shouldBe listOf("xx")
+        (statements[2] as Assignment).target.identifier!!.nameInSource shouldBe listOf("c64", "EXTCOL")
+        (statements[3] as Assignment).target.identifier!!.nameInSource shouldBe listOf("c64", "EXTCOL")
+        (statements[4] as Assignment).target.identifier!!.nameInSource shouldBe listOf("c64", "EXTCOL")
+        (statements[5] as Assignment).target.memoryAddress!!.addressExpression.constValue(result.program)!!.number shouldBe 53281.0
+        (statements[6] as Assignment).target.memoryAddress!!.addressExpression.constValue(result.program)!!.number shouldBe 53281.0
+        (statements[7] as Assignment).target.memoryAddress!!.addressExpression.constValue(result.program)!!.number shouldBe 53281.0
+    }
 })
diff --git a/compilerAst/src/prog8/ast/statements/AstStatements.kt b/compilerAst/src/prog8/ast/statements/AstStatements.kt
index b268c04ac..f591555e4 100644
--- a/compilerAst/src/prog8/ast/statements/AstStatements.kt
+++ b/compilerAst/src/prog8/ast/statements/AstStatements.kt
@@ -532,8 +532,7 @@ data class AssignTarget(var identifier: IdentifierReference?,
                 } else false
             }
             ident != null -> {
-                val decl = ident.targetVarDecl(definingModule.program) ?:
-                throw FatalAstException("invalid identifier ${ident.nameInSource}")
+                val decl = ident.targetVarDecl(definingModule.program) ?: throw FatalAstException("invalid identifier ${ident.nameInSource}")
                 return if (decl.type == VarDeclType.MEMORY && decl.value is NumericLiteral)
                     machine.isIOAddress((decl.value as NumericLiteral).number.toUInt())
                 else