diff --git a/internal/gengapic/gengapic.go b/internal/gengapic/gengapic.go
index 38e90dc1eb5..736b53db911 100644
--- a/internal/gengapic/gengapic.go
+++ b/internal/gengapic/gengapic.go
@@ -441,8 +441,8 @@ func (g *generator) insertDynamicRequestHeaders(m *descriptorpb.MethodDescriptor
 		return nil
 	}
 
-	g.printf(`routingHeaders := ""`)
-	g.printf("routingHeadersMap := make(map[string]string)")
+	g.printf(`var routingHeaders []string`)
+	g.printf("seen := make(map[string]bool)")
 	for i := range headers {
 		namedCaptureRegex := headers[i][0]
 		field := headers[i][1]
@@ -455,15 +455,14 @@ func (g *generator) insertDynamicRequestHeaders(m *descriptorpb.MethodDescriptor
 		}
 		// There could be an edge case where the request field is empty and the path template is a wildcard. In that case, we still don't want to send an empty header name.
 		g.printf("if reg := regexp.MustCompile(%q); reg.MatchString(%s) && len(%s) > 0 {", namedCaptureRegex, accessor, regexHelper)
-		g.printf("  routingHeadersMap[%q] = %s", headerName, regexHelper)
+		g.printf("  if !seen[%q] {", headerName)
+		g.printf("    routingHeaders = append(routingHeaders, fmt.Sprintf(\"%%s=%%s\", %q, %s))", headerName, regexHelper)
+		g.printf("    seen[%q] = true", headerName)
+		g.printf("  }")
 		g.printf("}")
 	}
-	g.printf("for headerName, headerValue := range routingHeadersMap {")
-	g.printf(`  routingHeaders = fmt.Sprintf("%%s%%s=%%s&", routingHeaders, headerName, headerValue)`)
-	g.printf("}")
-	g.printf(`routingHeaders = strings.TrimSuffix(routingHeaders, "&")`)
+	g.printf(`hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}`)
 	g.imports[pbinfo.ImportSpec{Path: "strings"}] = true
-	g.printf(`hds := []string{"x-goog-request-params", routingHeaders}`)
 	g.imports[pbinfo.ImportSpec{Path: "regexp"}] = true
 	return nil
 }
diff --git a/internal/gengapic/testdata/method_GetAnotherThing.want b/internal/gengapic/testdata/method_GetAnotherThing.want
index 1849b02789a..098b5595e9d 100644
--- a/internal/gengapic/testdata/method_GetAnotherThing.want
+++ b/internal/gengapic/testdata/method_GetAnotherThing.want
@@ -1,32 +1,49 @@
 func (c *fooGRPCClient) GetAnotherThing(ctx context.Context, req *mypackagepb.InputType, opts ...gax.CallOption) (*mypackagepb.OutputType, error) {
-	routingHeaders := ""
-	routingHeadersMap := make(map[string]string)
+	var routingHeaders []string
+	seen := make(map[string]bool)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
+		if !seen["other"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
+			seen["other"] = true
+		}
 	}
 	if reg := regexp.MustCompile("(?P<name>projects/[^/]+)/foos"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		routingHeadersMap["name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
+		if !seen["name"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "name", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
+			seen["name"] = true
+		}
 	}
 	if reg := regexp.MustCompile("(?P<foo_name>projects/[^/]+)/bars/[^/]+(?:/.*)?"); reg.MatchString(req.GetAnother()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])) > 0 {
-		routingHeadersMap["foo_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])
+		if !seen["foo_name"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "foo_name", url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])))
+			seen["foo_name"] = true
+		}
 	}
 	if reg := regexp.MustCompile("(?P<foo_name>projects/[^/]+/foos/[^/]+)/bars/[^/]+(?:/.*)?"); reg.MatchString(req.GetAnother()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])) > 0 {
-		routingHeadersMap["foo_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])
+		if !seen["foo_name"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "foo_name", url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])))
+			seen["foo_name"] = true
+		}
 	}
 	if reg := regexp.MustCompile("(?P<foo_name>.*)"); reg.MatchString(req.GetAnother()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])) > 0 {
-		routingHeadersMap["foo_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])
+		if !seen["foo_name"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "foo_name", url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])))
+			seen["foo_name"] = true
+		}
 	}
 	if reg := regexp.MustCompile("(?P<nested_name>.*)"); reg.MatchString(req.GetFieldName().GetNested()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])) > 0 {
-		routingHeadersMap["nested_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])
+		if !seen["nested_name"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "nested_name", url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])))
+			seen["nested_name"] = true
+		}
 	}
 	if reg := regexp.MustCompile("(?P<part_of_nested>projects/[^/]+)/bars"); reg.MatchString(req.GetFieldName().GetNested()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])) > 0 {
-		routingHeadersMap["part_of_nested"] = url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])
+		if !seen["part_of_nested"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "part_of_nested", url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])))
+			seen["part_of_nested"] = true
+		}
 	}
-	for headerName, headerValue := range routingHeadersMap {
-		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
-	}
-	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	hds := []string{"x-goog-request-params", routingHeaders}
+	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
 
 	hds = append(c.xGoogHeaders, hds...)
 	ctx = gax.InsertMetadataIntoOutgoingContext(ctx, hds...)
diff --git a/internal/gengapic/testdata/rest_HttpBodyRPC.want b/internal/gengapic/testdata/rest_HttpBodyRPC.want
index 691d0a492fc..57eba9716c6 100644
--- a/internal/gengapic/testdata/rest_HttpBodyRPC.want
+++ b/internal/gengapic/testdata/rest_HttpBodyRPC.want
@@ -12,16 +12,15 @@ func (c *fooRESTClient) HttpBodyRPC(ctx context.Context, req *foopb.Foo, opts ..
 	baseUrl.Path += fmt.Sprintf("/v1/foo")
 
 	// Build HTTP headers from client and context metadata.
-	routingHeaders := ""
-	routingHeadersMap := make(map[string]string)
+	var routingHeaders []string
+	seen := make(map[string]bool)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
-	}
-	for headerName, headerValue := range routingHeadersMap {
-		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
+		if !seen["other"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
+			seen["other"] = true
+		}
 	}
-	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	hds := []string{"x-goog-request-params", routingHeaders}
+	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
 
 	hds = append(c.xGoogHeaders, hds...)
 	hds = append(hds, "Content-Type", "application/json")
diff --git a/internal/gengapic/testdata/rest_ServerStreamRPC.want b/internal/gengapic/testdata/rest_ServerStreamRPC.want
index 1cc8d1d668c..180b7f52e20 100644
--- a/internal/gengapic/testdata/rest_ServerStreamRPC.want
+++ b/internal/gengapic/testdata/rest_ServerStreamRPC.want
@@ -12,16 +12,15 @@ func (c *fooRESTClient) ServerStreamRPC(ctx context.Context, req *foopb.Foo, opt
 	baseUrl.Path += fmt.Sprintf("/v1/foo")
 
 	// Build HTTP headers from client and context metadata.
-	routingHeaders := ""
-	routingHeadersMap := make(map[string]string)
+	var routingHeaders []string
+	seen := make(map[string]bool)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
-	}
-	for headerName, headerValue := range routingHeadersMap {
-		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
+		if !seen["other"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
+			seen["other"] = true
+		}
 	}
-	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	hds := []string{"x-goog-request-params", routingHeaders}
+	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
 
 	hds = append(c.xGoogHeaders, hds...)
 	hds = append(hds, "Content-Type", "application/json")
diff --git a/internal/gengapic/testdata/rest_UnaryRPC.want b/internal/gengapic/testdata/rest_UnaryRPC.want
index dbf6297b6b9..92eababa2d8 100644
--- a/internal/gengapic/testdata/rest_UnaryRPC.want
+++ b/internal/gengapic/testdata/rest_UnaryRPC.want
@@ -20,16 +20,15 @@ func (c *fooRESTClient) UnaryRPC(ctx context.Context, req *foopb.Foo, opts ...ga
 	baseUrl.RawQuery = params.Encode()
 
 	// Build HTTP headers from client and context metadata.
-	routingHeaders := ""
-	routingHeadersMap := make(map[string]string)
+	var routingHeaders []string
+	seen := make(map[string]bool)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
-	}
-	for headerName, headerValue := range routingHeadersMap {
-		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
+		if !seen["other"] {
+			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
+			seen["other"] = true
+		}
 	}
-	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
-	hds := []string{"x-goog-request-params", routingHeaders}
+	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
 
 	hds = append(c.xGoogHeaders, hds...)
 	hds = append(hds, "Content-Type", "application/json")