diff --git a/dictionary/dictionary.go b/dictionary/dictionary.go
index b04ece0..a4f459f 100644
--- a/dictionary/dictionary.go
+++ b/dictionary/dictionary.go
@@ -1,10 +1,18 @@
 package dictionary
 
-import "errors"
-
 type Dictionary map[string]string
 
-var ErrNotFound = errors.New("could not find the word you were looking for")
+const (
+	ErrNotFound   = DictionaryErr("could not find the word you were looking for")
+	ErrWordExists = DictionaryErr("word exists")
+)
+
+type DictionaryErr string
+
+// implements the Error interface, makes it an "error"
+func (e DictionaryErr) Error() string {
+	return string(e)
+}
 
 func (d Dictionary) Search(word string) (string, error) {
 	result, ok := d[word]
@@ -14,6 +22,15 @@ func (d Dictionary) Search(word string) (string, error) {
 	return result, nil
 }
 
-func (d Dictionary) Add(word, definition string) {
-	d[word] = definition
+func (d Dictionary) Add(word, definition string) error {
+	_, err := d.Search(word)
+	switch err {
+	case ErrNotFound:
+		d[word] = definition
+	case nil:
+		return ErrWordExists
+	default:
+		return err
+	}
+	return nil
 }
diff --git a/dictionary/dictionary_test.go b/dictionary/dictionary_test.go
index 4e35081..ef17f33 100644
--- a/dictionary/dictionary_test.go
+++ b/dictionary/dictionary_test.go
@@ -50,11 +50,23 @@ func assertDefinition(t testing.TB, dict Dictionary, word, def string) {
 }
 
 func TestAdd(t *testing.T) {
-	dict := Dictionary{}
+	t.Run("new word", func(t *testing.T) {
+		dict := Dictionary{}
 
-	word := "test"
-	def := "this is just a test"
+		word := "test"
+		def := "this is just a test"
 
-	dict.Add(word, def)
-	assertDefinition(t, dict, word, def)
+		dict.Add(word, def)
+		assertDefinition(t, dict, word, def)
+	})
+
+	t.Run("existing word", func(t *testing.T) {
+		word := "test"
+		def := "this is just a test"
+		dict := Dictionary{word: def}
+
+		err := dict.Add(word, def)
+		assertError(t, err, ErrWordExists)
+		assertDefinition(t, dict, word, def)
+	})
 }