diff --git a/config/cli.go b/config/cli.go
index 0977306..99db4f0 100644
--- a/config/cli.go
+++ b/config/cli.go
@@ -45,7 +45,7 @@ Flags:
   -m, -method       string    HTTP Method for the request (default %s)
   -b, -body         [string]  Body for the request (e.g. "body text")
   -p, -param        [string]  Parameter for the request (e.g. "key1=value1")
-  -H, -header       [string]  Header for the request (e.g. "key1: value1")
+  -H, -header       [string]  Header for the request (e.g. "key1:value1")
   -c, -cookie       [string]  Cookie for the request (e.g. "key1=value1")
   -x, -proxy        [string]  Proxy for the request (e.g. "http://proxy.example.com:8080")`
 
diff --git a/config/config.go b/config/config.go
index e92dff4..00a052f 100644
--- a/config/config.go
+++ b/config/config.go
@@ -233,4 +233,5 @@ func (config *Config) SetDefaults() {
 	if config.Yes == nil {
 		config.Yes = utils.ToPtr(DefaultYes)
 	}
+	config.Headers.SetIfNotExists("User-Agent", DefaultUserAgent)
 }
diff --git a/requests/request.go b/requests/request.go
index f65804a..5cb810a 100644
--- a/requests/request.go
+++ b/requests/request.go
@@ -166,9 +166,6 @@ func setRequestHeaders(req *fasthttp.Request, headers []types.KeyValue[string, s
 	for _, header := range headers {
 		req.Header.Add(header.Key, header.Value)
 	}
-	if req.Header.UserAgent() == nil {
-		req.Header.SetUserAgent(config.DefaultUserAgent)
-	}
 }
 
 // setRequestCookies adds the cookies of the given request with the provided key-value pairs.
diff --git a/types/headers.go b/types/headers.go
index 12684ec..3df8299 100644
--- a/types/headers.go
+++ b/types/headers.go
@@ -73,6 +73,15 @@ func (headers Headers) GetValue(key string) *[]string {
 	return nil
 }
 
+func (headers Headers) Has(key string) bool {
+	for i := range headers {
+		if headers[i].Key == key {
+			return true
+		}
+	}
+	return false
+}
+
 func (headers *Headers) UnmarshalJSON(b []byte) error {
 	var data []map[string]any
 	if err := json.Unmarshal(b, &data); err != nil {
@@ -137,3 +146,11 @@ func (headers *Headers) Set(value string) error {
 
 	return nil
 }
+
+func (headers *Headers) SetIfNotExists(key string, value string) bool {
+	if headers.Has(key) {
+		return false
+	}
+	*headers = append(*headers, KeyValue[string, []string]{Key: key, Value: []string{value}})
+	return true
+}