diff --git a/internal/gengapic/gengapic.go b/internal/gengapic/gengapic.go
index 527ed8343b8..42c7ff72e1c 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(`var routingHeaders []string`)
-	g.printf("seen := make(map[string]bool)")
+	g.printf(`routingHeaders := ""`)
+	g.printf("routingHeadersMap := make(map[string]string)")
 	for i := range headers {
 		namedCaptureRegex := headers[i][0]
 		field := headers[i][1]
@@ -455,14 +455,15 @@ 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("  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("  routingHeadersMap[%q] = %s", headerName, regexHelper)
 		g.printf("}")
 	}
-	g.printf(`hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}`)
+	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.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 098b5595e9d..1849b02789a 100644
--- a/internal/gengapic/testdata/method_GetAnotherThing.want
+++ b/internal/gengapic/testdata/method_GetAnotherThing.want
@@ -1,49 +1,32 @@
 func (c *fooGRPCClient) GetAnotherThing(ctx context.Context, req *mypackagepb.InputType, opts ...gax.CallOption) (*mypackagepb.OutputType, error) {
-	var routingHeaders []string
-	seen := make(map[string]bool)
+	routingHeaders := ""
+	routingHeadersMap := make(map[string]string)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		if !seen["other"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
-			seen["other"] = true
-		}
+		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
 	}
 	if reg := regexp.MustCompile("(?P<name>projects/[^/]+)/foos"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		if !seen["name"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "name", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
-			seen["name"] = true
-		}
+		routingHeadersMap["name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
 	}
 	if reg := regexp.MustCompile("(?P<foo_name>projects/[^/]+)/bars/[^/]+(?:/.*)?"); reg.MatchString(req.GetAnother()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])) > 0 {
-		if !seen["foo_name"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "foo_name", url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])))
-			seen["foo_name"] = true
-		}
+		routingHeadersMap["foo_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])
 	}
 	if reg := regexp.MustCompile("(?P<foo_name>projects/[^/]+/foos/[^/]+)/bars/[^/]+(?:/.*)?"); reg.MatchString(req.GetAnother()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])) > 0 {
-		if !seen["foo_name"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "foo_name", url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])))
-			seen["foo_name"] = true
-		}
+		routingHeadersMap["foo_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])
 	}
 	if reg := regexp.MustCompile("(?P<foo_name>.*)"); reg.MatchString(req.GetAnother()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])) > 0 {
-		if !seen["foo_name"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "foo_name", url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])))
-			seen["foo_name"] = true
-		}
+		routingHeadersMap["foo_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetAnother())[1])
 	}
 	if reg := regexp.MustCompile("(?P<nested_name>.*)"); reg.MatchString(req.GetFieldName().GetNested()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])) > 0 {
-		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
-		}
+		routingHeadersMap["nested_name"] = url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])
 	}
 	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 {
-		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
-		}
+		routingHeadersMap["part_of_nested"] = url.QueryEscape(reg.FindStringSubmatch(req.GetFieldName().GetNested())[1])
 	}
-	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
+	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 = 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 57eba9716c6..691d0a492fc 100644
--- a/internal/gengapic/testdata/rest_HttpBodyRPC.want
+++ b/internal/gengapic/testdata/rest_HttpBodyRPC.want
@@ -12,15 +12,16 @@ 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.
-	var routingHeaders []string
-	seen := make(map[string]bool)
+	routingHeaders := ""
+	routingHeadersMap := make(map[string]string)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		if !seen["other"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
-			seen["other"] = true
-		}
+		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
+	}
+	for headerName, headerValue := range routingHeadersMap {
+		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
 	}
-	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
+	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
+	hds := []string{"x-goog-request-params", 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 180b7f52e20..1cc8d1d668c 100644
--- a/internal/gengapic/testdata/rest_ServerStreamRPC.want
+++ b/internal/gengapic/testdata/rest_ServerStreamRPC.want
@@ -12,15 +12,16 @@ 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.
-	var routingHeaders []string
-	seen := make(map[string]bool)
+	routingHeaders := ""
+	routingHeadersMap := make(map[string]string)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		if !seen["other"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
-			seen["other"] = true
-		}
+		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
+	}
+	for headerName, headerValue := range routingHeadersMap {
+		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
 	}
-	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
+	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
+	hds := []string{"x-goog-request-params", 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 92eababa2d8..dbf6297b6b9 100644
--- a/internal/gengapic/testdata/rest_UnaryRPC.want
+++ b/internal/gengapic/testdata/rest_UnaryRPC.want
@@ -20,15 +20,16 @@ func (c *fooRESTClient) UnaryRPC(ctx context.Context, req *foopb.Foo, opts ...ga
 	baseUrl.RawQuery = params.Encode()
 
 	// Build HTTP headers from client and context metadata.
-	var routingHeaders []string
-	seen := make(map[string]bool)
+	routingHeaders := ""
+	routingHeadersMap := make(map[string]string)
 	if reg := regexp.MustCompile("(.*)"); reg.MatchString(req.GetOther()) && len(url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])) > 0 {
-		if !seen["other"] {
-			routingHeaders = append(routingHeaders, fmt.Sprintf("%s=%s", "other", url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])))
-			seen["other"] = true
-		}
+		routingHeadersMap["other"] = url.QueryEscape(reg.FindStringSubmatch(req.GetOther())[1])
+	}
+	for headerName, headerValue := range routingHeadersMap {
+		routingHeaders = fmt.Sprintf("%s%s=%s&", routingHeaders, headerName, headerValue)
 	}
-	hds := []string{"x-goog-request-params", strings.Join(routingHeaders, "&")}
+	routingHeaders = strings.TrimSuffix(routingHeaders, "&")
+	hds := []string{"x-goog-request-params", routingHeaders}
 
 	hds = append(c.xGoogHeaders, hds...)
 	hds = append(hds, "Content-Type", "application/json")