diff --git a/Source/Base.lproj/Localizable.strings b/Source/Base.lproj/Localizable.strings
index 3f906806..6a30f895 100644
--- a/Source/Base.lproj/Localizable.strings
+++ b/Source/Base.lproj/Localizable.strings
@@ -90,3 +90,5 @@
 "Cursor is after \"%@\"." = "Cursor is after \"%@\".";
 
 "Cursor is between \"%@\" and \"%@\"." = "Cursor is between \"%@\" and \"%@\".";
+
+"You are now selecting \"%@\". The phrase already exists." = "You are now selecting \"%@\". The phrase already exists.";
diff --git a/Source/InputState.swift b/Source/InputState.swift
index 23f2f39c..61217c47 100644
--- a/Source/InputState.swift
+++ b/Source/InputState.swift
@@ -185,6 +185,16 @@ class InputState: NSObject {
             } else if (markedRange.length > kMaxMarkRangeLength) {
                 return String(format: NSLocalizedString("You are now selecting \"%@\". A phrase cannot be longer than %d characters.", comment: ""), text, kMaxMarkRangeLength)
             }
+
+            let (exactBegin, _) = (composingBuffer as NSString).characterIndex(from: markedRange.location)
+            let (exactEnd, _) = (composingBuffer as NSString).characterIndex(from: markedRange.location + markedRange.length)
+            let selectedReadings = readings[exactBegin..<exactEnd]
+            let joined = selectedReadings.joined(separator: "-")
+            let exist = LanguageModelManager.checkIfExist(userPhrase: text, key: joined)
+            if exist {
+                return String(format: NSLocalizedString("You are now selecting \"%@\". The phrase already exists.", comment: ""), text)
+            }
+
             return String(format: NSLocalizedString("You are now selecting \"%@\". Press enter to add a new phrase.", comment: ""), text)
         }
 
@@ -238,9 +248,18 @@ class InputState: NSObject {
             if composingBuffer.count != readings.count {
                 return false
             }
-
-            return markedRange.length >= kMinMarkRangeLength &&
-                markedRange.length <= kMaxMarkRangeLength
+            if  markedRange.length < kMinMarkRangeLength {
+                return false
+            }
+            if markedRange.length > kMaxMarkRangeLength {
+                return false
+            }
+            let text = (composingBuffer as NSString).substring(with: markedRange)
+            let (exactBegin, _) = (composingBuffer as NSString).characterIndex(from: markedRange.location)
+            let (exactEnd, _) = (composingBuffer as NSString).characterIndex(from: markedRange.location + markedRange.length)
+            let selectedReadings = readings[exactBegin..<exactEnd]
+            let joined = selectedReadings.joined(separator: "-")
+            return LanguageModelManager.checkIfExist(userPhrase: text, key: joined) == false
         }
 
         @objc var userPhrase: String {
diff --git a/Source/LanguageModelManager.h b/Source/LanguageModelManager.h
index ad75814d..8fdcfbc5 100644
--- a/Source/LanguageModelManager.h
+++ b/Source/LanguageModelManager.h
@@ -33,6 +33,8 @@ NS_ASSUME_NONNULL_BEGIN
 + (void)loadUserPhraseReplacement;
 + (void)setupDataModelValueConverter;
 + (BOOL)checkIfUserLanguageModelFilesExist;
+
++ (BOOL)checkIfUserPhraseExist:(NSString *)userPhrase key:(NSString *)key NS_SWIFT_NAME(checkIfExist(userPhrase:key:));
 + (BOOL)writeUserPhrase:(NSString *)userPhrase;
 
 @property (class, readonly, nonatomic) NSString *dataFolderPath;
diff --git a/Source/LanguageModelManager.mm b/Source/LanguageModelManager.mm
index df6b4019..59c31b52 100644
--- a/Source/LanguageModelManager.mm
+++ b/Source/LanguageModelManager.mm
@@ -193,6 +193,19 @@ static void LTLoadAssociatedPhrases(McBopomofoLM &lm)
     return YES;
 }
 
++ (BOOL)checkIfUserPhraseExist:(NSString *)userPhrase key:(NSString *)key NS_SWIFT_NAME(checkIfExist(userPhrase:key:))
+{
+    string unigramKey = string(key.UTF8String);
+    vector<Unigram> unigrams = gLanguageModelMcBopomofo.unigramsForKey(unigramKey);
+    string userPhraseString = string(userPhrase.UTF8String);
+    for (auto unigram: unigrams) {
+        if (unigram.keyValue.value == userPhraseString) {
+            return YES;
+        }
+    }
+    return NO;
+}
+
 + (BOOL)writeUserPhrase:(NSString *)userPhrase
 {
     if (![self checkIfUserLanguageModelFilesExist]) {
diff --git a/Source/en.lproj/Localizable.strings b/Source/en.lproj/Localizable.strings
index 3f906806..acaee5b1 100644
--- a/Source/en.lproj/Localizable.strings
+++ b/Source/en.lproj/Localizable.strings
@@ -90,3 +90,6 @@
 "Cursor is after \"%@\"." = "Cursor is after \"%@\".";
 
 "Cursor is between \"%@\" and \"%@\"." = "Cursor is between \"%@\" and \"%@\".";
+
+"You are now selecting \"%@\". The phrase already exists." = "You are now selecting \"%@\". The phrase already exists.";
+
diff --git a/Source/zh-Hant.lproj/Localizable.strings b/Source/zh-Hant.lproj/Localizable.strings
index 3bcd325e..36c4370c 100644
--- a/Source/zh-Hant.lproj/Localizable.strings
+++ b/Source/zh-Hant.lproj/Localizable.strings
@@ -47,11 +47,11 @@
 
 "Use Half-Width Punctuations" = "使用半型標點符號";
 
-"You are now selecting \"%@\". You can add a phrase with two or more characters." = "您目前選擇了 \"%@\"。請選擇兩個字以上，才能加入使用者詞彙。";
+"You are now selecting \"%@\". You can add a phrase with two or more characters." = "您目前選擇了「%@」。請選擇兩個字以上，才能加入使用者詞彙。";
 
-"You are now selecting \"%@\". Press enter to add a new phrase." = "您目前選擇了 \"%@\"。按下 Enter 就可以加入到使用者詞彙中。";
+"You are now selecting \"%@\". Press enter to add a new phrase." = "您目前選擇了「%@」。按下 Enter 就可以加入到使用者詞彙中。";
 
-"You are now selecting \"%@\". A phrase cannot be longer than %d characters." = "您目前選擇了 \"%@\"。自訂詞彙不能超過 %d 個字元。";
+"You are now selecting \"%@\". A phrase cannot be longer than %d characters." = "您目前選擇了「%@」。自訂詞彙不能超過 %d 個字元。";
 
 "Chinese conversion on" = "已經切換到簡體中文模式";
 
@@ -90,3 +90,5 @@
 "Cursor is after \"%@\"." = "游標正在「%@」後方";
 
 "Cursor is between \"%@\" and \"%@\"." = "游標正在「%@」與「%@」之間";
+
+"You are now selecting \"%@\". The phrase already exists." = "您目前選擇了「%@」，這個詞彙已經存在了";