diff --git a/background.js b/background.js
index 271a1f4..73203cc 100644
--- a/background.js
+++ b/background.js
@@ -116,6 +116,12 @@ function storeInBrowserStorage(item, callback_function) {
     chrome.storage.local.set(item, callback_function);
 }
 
+function hasCookieKey(cookie, key) {
+	let cookieArray = cookie.value.split(';').filter((e) => e.trim().length > 0);
+    let cookieKeyIndex = cookieArray.findIndex((kv) => kv.trim().startsWith(key + '='));
+    return cookieKeyIndex !== -1;
+}
+
 /*
  * This function set a key-value pair in HTTP header "Cookie",
  *   and returns the value of HTTP header after modification.
@@ -230,7 +236,7 @@ function rewriteRequestHeader(e) {
                             'Delete request header :  name=' + to_modify.header_name.toLowerCase() + ' for url ' + e.url
                         );
                 }
-            } else if (to_modify.action === 'cookie_add_or_modify') {
+            } else if (to_modify.action === 'cookie_add') {
                 let header_cookie = e.requestHeaders.find((header) => header.name.toLowerCase() === 'cookie');
                 let new_cookie = cookie_keyvalues_set(
                     header_cookie === undefined ? '' : header_cookie.value,
@@ -241,16 +247,33 @@ function rewriteRequestHeader(e) {
                     e.requestHeaders.push({name: 'Cookie', value: new_cookie});
                     if (config.debug_mode)
                         log(
-                            'cookie_add_or_modify.req new_header : name=Cookie,value=' +
+                            'cookie_add.req new_header : name=Cookie,value=' +
                                 new_cookie +
                                 ' for url ' +
                                 e.url
                         );
-                } else {
+                } else if ( ! hasCookieKey(header_cookie, to_modify.header_name)) {
+                    header_cookie.value = new_cookie;
+                    if (config.debug_mode)
+                        log(
+                            'cookie_modify.req modify_header : name=Cookie,value=' +
+                                new_cookie +
+                                ' for url ' +
+                                e.url
+                        );
+                }
+            } else if (to_modify.action === 'cookie_modify') {
+                let header_cookie = e.requestHeaders.find((header) => header.name.toLowerCase() === 'cookie');
+                let new_cookie = cookie_keyvalues_set(
+                    header_cookie === undefined ? '' : header_cookie.value,
+                    to_modify.header_name,
+                    to_modify.header_value
+                );
+                if (header_cookie != undefined) {
                     header_cookie.value = new_cookie;
                     if (config.debug_mode)
                         log(
-                            'cookie_add_or_modify.req modify_header : name=Cookie,value=' +
+                            'cookie_modify.req modify_header : name=Cookie,value=' +
                                 new_cookie +
                                 ' for url ' +
                                 e.url
@@ -346,7 +369,7 @@ function rewriteResponseHeader(e) {
                                 e.url
                         );
                 }
-            } else if (to_modify.action === 'cookie_add_or_modify') {
+            } else if (to_modify.action === 'cookie_add') {
                 let header_cookie = e.responseHeaders.find(
                     (header) =>
                         header.name.toLowerCase() === 'set-cookie' &&
@@ -362,21 +385,36 @@ function rewriteResponseHeader(e) {
                 );
                 if (header_cookie === undefined) {
                     log(
-                        "SimpleModifyHeaders.Warning: you're using cookie_add_or_modify in Response. While adding new cookie in response, this plugin only generates `Set-Cookie: cookie-name=cookie-value `, without ANY additional attributes. Add a `Set-Cookie` header if you need them. "
+                        "SimpleModifyHeaders.Warning: you're using cookie_add in Response. While adding new cookie in response, this plugin only generates `Set-Cookie: cookie-name=cookie-value `, without ANY additional attributes. Add a `Set-Cookie` header if you need them. "
                     );
                     e.responseHeaders.push({name: 'Set-Cookie', value: new_header_value});
                     if (config.debug_mode)
                         log(
-                            'cookie_add_or_modify.resp new_header : name=Cookie,value=' +
+                            'cookie_add.resp new_header : name=Cookie,value=' +
                                 new_header_value +
                                 ' for url ' +
                                 e.url
                         );
-                } else {
+                }
+            } else if (to_modify.action === 'cookie_modify') {
+                let header_cookie = e.responseHeaders.find(
+                    (header) =>
+                        header.name.toLowerCase() === 'set-cookie' &&
+                        header.value
+                            .toLowerCase()
+                            .trim()
+                            .startsWith(to_modify.header_name.toLowerCase() + '=')
+                );
+                let new_header_value = set_cookie_modify_cookie_value(
+                    header_cookie === undefined ? '' : header_cookie.value,
+                    to_modify.header_name,
+                    to_modify.header_value
+                );
+                if (header_cookie != undefined) {
                     header_cookie.value = new_header_value;
                     if (config.debug_mode)
                         log(
-                            'cookie_add_or_modify.resp modify_header : name=Cookie,value=' +
+                            'cookie_modify.resp modify_header : name=Cookie,value=' +
                                 new_header_value +
                                 ' for url ' +
                                 e.url
diff --git a/manifestV3..json b/manifestV3.json
similarity index 95%
rename from manifestV3..json
rename to manifestV3.json
index 65f94a9..3d95ecd 100644
--- a/manifestV3..json
+++ b/manifestV3.json
@@ -25,4 +25,4 @@
   "background": {
     "service_worker": "service-worker.js"
   }
-}
\ No newline at end of file
+}
diff --git a/popup/config.js b/popup/config.js
index 832f7fa..303442c 100644
--- a/popup/config.js
+++ b/popup/config.js
@@ -190,7 +190,8 @@ function appendLine(url_contains, action, header_name, header_value, comment, ap
     if (!useManifestV3) {
         // Not available in Manifest V3
         html += `
-        <option value="cookie_add_or_modify">Cookie Add/Modify</option>
+        <option value="cookie_add">Cookie Add</option>
+	    <option value="cookie_modify">Cookie Modify</option>
         <option value="cookie_delete">Cookie Delete</option> `;
     }
     html += `