diff --git a/src/main/java/de/neemann/digital/lang/Lang.java b/src/main/java/de/neemann/digital/lang/Lang.java
index f70df61d8..8440fce06 100644
--- a/src/main/java/de/neemann/digital/lang/Lang.java
+++ b/src/main/java/de/neemann/digital/lang/Lang.java
@@ -5,6 +5,7 @@ import de.neemann.gui.language.Language;
 import de.neemann.gui.language.Resources;
 
 import java.text.MessageFormat;
+import java.util.Arrays;
 import java.util.Locale;
 import java.util.prefs.Preferences;
 
@@ -90,6 +91,7 @@ public final class Lang {
         else
             currentLanguage = new Language("en");
     }
+
     /**
      * Only used for generation of documentation
      *
@@ -111,9 +113,9 @@ public final class Lang {
             // if the 'testdata' system property is set, let the test fail!
             // If we are in production usage, don't let the program  crash, simply return the key itself instead!
             if (System.getProperty("testdata") != null)
-                throw new Error("missing laguage key '" + key + "'");
+                throw new Error("missing language key '" + key + "'");
 
-            return key;
+            return key + " " + Arrays.asList(params).toString();
         }
     }
 
diff --git a/src/main/java/de/neemann/digital/testing/TestResult.java b/src/main/java/de/neemann/digital/testing/TestResult.java
index 7c41fef55..6cb370a5a 100644
--- a/src/main/java/de/neemann/digital/testing/TestResult.java
+++ b/src/main/java/de/neemann/digital/testing/TestResult.java
@@ -61,14 +61,14 @@ public class TestResult {
             final int index = getIndexOf(s.getName());
             if (index >= 0) {
                 inputs.add(new TestSignal(index, s.getValue()));
-                usedSignals.add(s.getName());
+                addTo(usedSignals, s.getName());
             }
         }
         for (Clock c : model.getClocks()) {
             final int index = getIndexOf(c.getLabel());
             if (index >= 0) {
                 inputs.add(new TestSignal(index, c.getClockOutput()));
-                usedSignals.add(c.getLabel());
+                addTo(usedSignals, c.getLabel());
             }
         }
 
@@ -77,7 +77,7 @@ public class TestResult {
             final int index = getIndexOf(s.getName());
             if (index >= 0) {
                 outputs.add(new TestSignal(index, s.getValue()));
-                usedSignals.add(s.getName());
+                addTo(usedSignals, s.getName());
             }
         }
 
@@ -107,6 +107,12 @@ public class TestResult {
         return this;
     }
 
+    private void addTo(HashSet<String> signals, String name) throws TestingDataException {
+        if (signals.contains(name))
+            throw new TestingDataException(Lang.get("err_nameUsedTwice_N", name));
+        signals.add(name);
+    }
+
     private void checkRow(Model model, Value[] row) {
         Value[] res = new Value[row.length];
 
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 9ac0cd6fd..eec58085d 100644
--- a/src/main/java/de/neemann/digital/testing/parser/Parser.java
+++ b/src/main/java/de/neemann/digital/testing/parser/Parser.java
@@ -54,7 +54,10 @@ public class Parser {
             Tokenizer.Token token = tok.simpleIdent();
             switch (token) {
                 case IDENT:
-                    names.add(tok.getIdent());
+                    final String name = tok.getIdent();
+                    if (names.contains(name))
+                        throw new ParserException(Lang.get("err_nameUsedTwice_N", name));
+                    names.add(name);
                     break;
                 case EOL:
                     return;
diff --git a/src/main/resources/lang/lang_de.xml b/src/main/resources/lang/lang_de.xml
index 44fd7ad89..db98fd72b 100644
--- a/src/main/resources/lang/lang_de.xml
+++ b/src/main/resources/lang/lang_de.xml
@@ -424,7 +424,8 @@ Die gesammte Speichergröße beträgt damit damit dx*dy*2 Speicherworte.</string
     <string name="elem_LedMatrix_pin_c-addr">Die Nummer der aktuellen Spalte, dessen Zustand gerade am anderen Eingang anliegt.</string>
 
     <string name="elem_TransGate">Transmissionsgatter</string>
-    <string name="elem_TransGate_tt">Ein reales Transmissionsgatter ist aus nur zwei Transistoren aufgebaut.</string>
+    <string name="elem_TransGate_tt">Ein reales Transmissionsgatter ist aus nur zwei Transistoren aufgebaut.
+        Daher wird es oft eingesetzt um Transistoren einzusparen.</string>
     <string name="elem_TransGate_pin_A">Eingang A</string>
     <string name="elem_TransGate_pin_B">Eingang_B</string>
     <string name="elem_TransGate_pin_S">Steuereingang</string>
@@ -523,6 +524,7 @@ Sind evtl. die Namen der Variablen nicht eindeutig?</string>
     <string name="err_toManyBits_Found_N0_maxIs_N1">Es sind nur {1} Bits erlaubt, es sind jedoch {0} Bits angegeben!</string>
     <string name="err_MultiBitFlipFlopFound">Es sind keine Flipflops mit mehr als einem Bit erlaubt!</string>
     <string name="err_invalidTransmissionGateState">Die Steuereingänge eines Transmission-Gates müssen invertiert beschaltet werden!</string>
+    <string name="err_nameUsedTwice_N">Signal {0} wurde mehrfach verwendet!</string>
 
     <string name="key_AddrBits">Adress-Bits</string>
     <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 ec20fc728..a3c852453 100644
--- a/src/main/resources/lang/lang_en.xml
+++ b/src/main/resources/lang/lang_en.xml
@@ -414,7 +414,8 @@
     <string name="elem_LedMatrix_pin_c-addr">The number of the current column whose state is currently visible at the other input.</string>
 
     <string name="elem_TransGate">Transmission-Gate</string>
-    <string name="elem_TransGate_tt">A real transmission-gate is build from only two transistors.</string>
+    <string name="elem_TransGate_tt">A real transmission-gate is build from only two transistors.
+        Therefore, it is often used to save transistors during implementation on silicon.</string>
     <string name="elem_TransGate_pin_A">input A</string>
     <string name="elem_TransGate_pin_B">input B</string>
     <string name="elem_TransGate_pin_S">control input.</string>
@@ -513,6 +514,7 @@ The names of the variables may not be unique.</string>
     <string name="err_toManyBits_Found_N0_maxIs_N1">Only {1} bits allowed, but {0} bits found!</string>
     <string name="err_MultiBitFlipFlopFound">Flipflops with more then one bits are not allowed!</string>
     <string name="err_invalidTransmissionGateState">The two control inputs of a transmission gate must be inverted!</string>
+    <string name="err_nameUsedTwice_N">Signal {0} is used twice!</string>
 
     <string name="key_AddrBits">Address Bits</string>
     <string name="key_AddrBits_tt">Number of address bits used.</string>