diff --git a/src/ArgsParser/ArgsParser.java b/src/ArgsParser/ArgsParser.java
index cdec669..2919fc9 100644
--- a/src/ArgsParser/ArgsParser.java
+++ b/src/ArgsParser/ArgsParser.java
@@ -60,6 +60,8 @@ public class ArgsParser {
     private final String[] args;
     private final Map<String, Parameter<?>> parameterMap = new HashMap<>();
     private final Set<Parameter<?>> mandatoryParameters = new HashSet<>();
+    private final Set<String> fullFlags = new HashSet<>();
+    private final Set<String> shortFlags = new HashSet<>();
     protected boolean parseArgsWasCalled = false;
     private int longestFlagSize = 0;
     private int longestShortFlag = 0;
@@ -92,6 +94,11 @@ private <T> Parameter<T> createParameter(String fullFlag,
                                              boolean isMandatory,
                                              T defaultValue) {
 
+        // check if the flag names are already used
+        if (fullFlags.contains(fullFlag)) throw new IllegalArgumentException("Flag already exists: " + fullFlag);
+        if (shortFlags.contains(shortFlag)) throw new IllegalArgumentException("Flag already exists: " + shortFlag);
+
+        // create new parameter instance
         Parameter<T> parameter = new Parameter<T>(makeFlag(fullFlag, false),
                                                   makeFlag(shortFlag, true),
                                                   description, type, isMandatory, this);
@@ -100,17 +107,22 @@ private <T> Parameter<T> createParameter(String fullFlag,
             parameter.setDefault(defaultValue);
         }
 
+        // add parameter to the map
         parameterMap.put(parameter.getFullFlag(), parameter);
         if (parameter.getShortFlag() != null) parameterMap.put(parameter.getShortFlag(), parameter);
 
+        // check for the lengths of the name
         int nameSize = parameter.getFullFlag().length();
         if (longestFlagSize < nameSize) longestFlagSize = nameSize;
 
-        if (parameter.getShortFlag() != null) {
-            int shortSize = parameter.getShortFlag().length();
-            if (longestShortFlag < shortSize) longestShortFlag = shortSize;
-        }
+        int shortSize = parameter.getShortFlag().length();
+        if (longestShortFlag < shortSize) longestShortFlag = shortSize;
+
+        // add names to the name sets
+        fullFlags.add(fullFlag);
+        shortFlags.add(shortFlag);
 
+        // add to mandatory parameters if parameter is mandatory
         if (parameter.isMandatory()) mandatoryParameters.add(parameter);
 
         return parameter;
diff --git a/src/test/TestArgsParser.java b/src/test/TestArgsParser.java
index 730420f..ff9d04b 100644
--- a/src/test/TestArgsParser.java
+++ b/src/test/TestArgsParser.java
@@ -521,4 +521,16 @@ public void testBool2() {
         Boolean result = bool.getArgument();
         assertFalse(result);
     }
+
+    @Test
+    public void testSameFlagNameException() {
+        ArgsParser parser = new ArgsParser(new String[] {"--file", "file.txt", "--file", "file2.txt"});
+        Parameter<String> file = parser.addStringParameter("file", "f", "descr", true);
+        try {
+            Parameter<String> file2 = parser.addStringParameter("file", "f2", "descr", true);
+            parser.parseArgs();
+        } catch (Exception e) {
+            assertEquals("Flag already exists: file", e.getMessage());
+        }
+    }
 }