diff --git a/src/main/java/de/neemann/digital/testing/parser/Parser.java b/src/main/java/de/neemann/digital/testing/parser/Parser.java
index 49ecbc283..6b549e056 100644
--- a/src/main/java/de/neemann/digital/testing/parser/Parser.java
+++ b/src/main/java/de/neemann/digital/testing/parser/Parser.java
@@ -113,7 +113,7 @@ public class Parser {
                     expect(Tokenizer.Token.EQUAL);
                     final Expression sigExpression = parseExpression();
                     expect(Tokenizer.Token.SEMICOLON);
-                    virtualSignals.add(new VirtualSignal(sigName, sigExpression));
+                    addVirtualSignal(new VirtualSignal(sigName, sigExpression));
                     break;
                 case END:
                     tok.consume();
@@ -158,6 +158,13 @@ public class Parser {
         }
     }
 
+    private void addVirtualSignal(VirtualSignal vs) throws ParserException {
+        for (VirtualSignal v : virtualSignals)
+            if (v.getName().equals(vs.getName()))
+                throw new ParserException(Lang.get("err_virtualSignal_N_DeclaredTwiceInLine_N", vs.getName(), tok.getLine()));
+        virtualSignals.add(vs);
+    }
+
     private LineEmitter parseSingleRow() throws IOException, ParserException {
         LineEmitterSimple line = null;
         while (true) {
diff --git a/src/main/resources/lang/lang_de.xml b/src/main/resources/lang/lang_de.xml
index fbd2fb2b6..1da90dc66 100644
--- a/src/main/resources/lang/lang_de.xml
+++ b/src/main/resources/lang/lang_de.xml
@@ -1152,6 +1152,7 @@ Sind evtl. die Namen der Variablen nicht eindeutig?</string>
 
     <string name="err_vgaModeNotDetected_N">Videomodus wurde nicht erkannt ({0})</string>
     <string name="err_ROM_noFileGivenToLoad">Es ist kein Dateiname für das automatische Neuladen verfügbar!</string>
+    <string name="err_virtualSignal_N_DeclaredTwiceInLine_N">Virtuelles Signal {0} doppelt definiert in Zeile {1}!</string>
 
     <string name="key_AddrBits">Adress-Bits</string><!-- ROM, RAMDualPort, RAMSinglePort, RAMSinglePortSel, EEPROM -->
     <string name="key_AddrBits_tt">Anzahl der Adress-Bits, die verwendet werden.</string>
diff --git a/src/main/resources/lang/lang_en.xml b/src/main/resources/lang/lang_en.xml
index 8ce1d3a00..64bd458d0 100644
--- a/src/main/resources/lang/lang_en.xml
+++ b/src/main/resources/lang/lang_en.xml
@@ -1161,6 +1161,7 @@
     <string name="err_inGenericInitCode">Error in the analysis of the generic initialization code.</string>
 
     <string name="err_ROM_noFileGivenToLoad">There is no file name available for the automatic reload!</string>
+    <string name="err_virtualSignal_N_DeclaredTwiceInLine_N">Virtual signal {0} declared twice in line {1}!</string>
 
     <string name="key_AddrBits">Address Bits</string><!-- ROM, RAMDualPort, RAMSinglePort, RAMSinglePortSel, EEPROM -->
     <string name="key_AddrBits_tt">Number of address bits used.</string>