diff --git a/go.mod b/go.mod
index 9f85ef1..8325db9 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module github.com/RambIing/urlValues
 
-go 1.16
+go 1.18
diff --git a/urlValues.go b/urlValues.go
index 254b1b2..1d7ab2e 100644
--- a/urlValues.go
+++ b/urlValues.go
@@ -6,6 +6,7 @@
 package urlValues
 
 import (
+	"slices"
 	"sort"
 	"strings"
 )
@@ -75,7 +76,9 @@ func (v Values) EncodeWithOrder() string {
 
 func (v Values) Add(key, value string) {
 	v[key] = append(v[key], value)
-	v[OrderKey] = append(v[OrderKey], key)
+	if !slices.Contains(v[OrderKey], key) {
+		v[OrderKey] = append(v[OrderKey], key)
+	}
 }
 
 func (v Values) Del(key string) {
diff --git a/urlValues_test.go b/urlValues_test.go
index ee6d5dc..8b60200 100644
--- a/urlValues_test.go
+++ b/urlValues_test.go
@@ -8,39 +8,52 @@ func getValues() Values {
 	values := Values{}
 	values.Add("xyz", "abc")
 	values.Add("abc", "xyz")
+	values.Add("foo", "bar")
+	values.Add("foo", "baa")
+	values.Add("foo", "baz")
 	return values
 }
 
 func TestValues_Encode(t *testing.T) {
 	values := getValues()
 
-	if values.Encode() != "abc=xyz&xyz=abc" {
-		t.FailNow()
+	encoded := values.Encode()
+
+	if encoded != "abc=xyz&foo=bar&foo=baa&foo=baz&xyz=abc" {
+		t.Fatalf("failed to encode values. got: %s", encoded)
 	}
 
 	values.Del("xyz")
 
-	if values.EncodeWithOrder() != "abc=xyz" {
-		t.Fatalf("failed to delete item")
+	encoded = values.Encode()
+
+	if encoded != "abc=xyz&foo=bar&foo=baa&foo=baz" {
+		t.Fatalf("failed to delete item. got: %s", encoded)
 	}
 }
 
 func TestValues_EncodeWithOrder(t *testing.T) {
 	values := getValues()
 
-	if values.EncodeWithOrder() != "xyz=abc&abc=xyz" {
-		t.FailNow()
+	encoded := values.EncodeWithOrder()
+
+	if encoded != "xyz=abc&abc=xyz&foo=bar&foo=baa&foo=baz" {
+		t.Fatalf("failed to encode values with order. got: %s", encoded)
 	}
 
-	values[OrderKey] = []string{"abc", "xyz"}
+	values[OrderKey] = []string{"abc", "xyz", "foo"}
 
-	if values.EncodeWithOrder() != "abc=xyz&xyz=abc" {
-		t.Fatalf("failed to manually rearrange order")
+	encoded = values.EncodeWithOrder()
+
+	if encoded != "abc=xyz&xyz=abc&foo=bar&foo=baa&foo=baz" {
+		t.Fatalf("failed to encode values with new order. got: %s", encoded)
 	}
 
 	values.Del("abc")
 
-	if values.EncodeWithOrder() != "xyz=abc" && len(values[OrderKey]) != 1 {
-		t.Fatalf("failed to fully delete item")
+	encoded = values.EncodeWithOrder()
+
+	if encoded != "xyz=abc&foo=bar&foo=baa&foo=baz" {
+		t.Fatalf("failed to delete item. got: %s", encoded)
 	}
 }