@@ -231,7 +231,7 @@ func TestMerge(t *testing.T) {

 	volumesUser := make(map[string]struct{})

 	volumesUser["/test3"] = struct{}{}

 	configUser := &Config{

-		Dns:       []string{"3.3.3.3"},

+		Dns:       []string{"2.2.2.2", "3.3.3.3"},

 		PortSpecs: []string{"3333:2222", "3333:3333"},

 		Env:       []string{"VAR2=3", "VAR3=3"},

 		Volumes:   volumesUser,

@@ -97,8 +97,15 @@ func Merge(userConf, imageConf *Config) error {

 	if userConf.Dns == nil || len(userConf.Dns) == 0 {

 		userConf.Dns = imageConf.Dns

 	} else {

-		//duplicates aren't an issue here

-		userConf.Dns = append(userConf.Dns, imageConf.Dns...)

+		dnsSet := make(map[string]struct{}, len(userConf.Dns))

+		for _, dns := range userConf.Dns {

+			dnsSet[dns] = struct{}{}

+		}

+		for _, dns := range imageConf.Dns {

+			if _, exists := dnsSet[dns]; !exists {

+				userConf.Dns = append(userConf.Dns, dns)

+			}

+		}

 	}

 	if userConf.Entrypoint == nil || len(userConf.Entrypoint) == 0 {

 		userConf.Entrypoint = imageConf.Entrypoint