diff --git a/control.go b/control.go
index d1d459f5..ada72697 100644
--- a/control.go
+++ b/control.go
@@ -559,15 +559,10 @@ func handleFilteringAddURL(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Check for duplicates
-	config.RLock()
-	for i := range config.Filters {
-		if config.Filters[i].URL == f.URL {
-			config.RUnlock()
-			httpError(w, http.StatusBadRequest, "Filter URL already added -- %s", f.URL)
-			return
-		}
+	if filterExists(f.URL) {
+		httpError(w, http.StatusBadRequest, "Filter URL already added -- %s", f.URL)
+		return
 	}
-	config.RUnlock()
 
 	// Set necessary properties
 	f.ID = assignUniqueFilterID()
@@ -597,20 +592,11 @@ func handleFilteringAddURL(w http.ResponseWriter, r *http.Request) {
 
 	// URL is deemed valid, append it to filters, update config, write new filter file and tell dns to reload it
 	// TODO: since we directly feed filters in-memory, revisit if writing configs is always necessary
-	config.Lock()
-
-	// Check for duplicates
-	for i := range config.Filters {
-		if config.Filters[i].URL == f.URL {
-			config.Unlock()
-			httpError(w, http.StatusBadRequest, "Filter URL already added -- %s", f.URL)
-			return
-		}
+	if !filterAdd(f) {
+		httpError(w, http.StatusBadRequest, "Filter URL already added -- %s", f.URL)
+		return
 	}
 
-	config.Filters = append(config.Filters, f)
-	config.Unlock()
-
 	err = writeAllConfigs()
 	if err != nil {
 		httpError(w, http.StatusInternalServerError, "Couldn't write config file: %s", err)
diff --git a/filter.go b/filter.go
index 6dedadc1..c294b3b5 100644
--- a/filter.go
+++ b/filter.go
@@ -72,6 +72,38 @@ func filterEnable(url string, enable bool) bool {
 	return r
 }
 
+// Return TRUE if a filter with this URL exists
+func filterExists(url string) bool {
+	r := false
+	config.RLock()
+	for i := range config.Filters {
+		if config.Filters[i].URL == url {
+			r = true
+			break
+		}
+	}
+	config.RUnlock()
+	return r
+}
+
+// Add a filter
+// Return FALSE if a filter with this URL exists
+func filterAdd(f filter) bool {
+	config.Lock()
+
+	// Check for duplicates
+	for i := range config.Filters {
+		if config.Filters[i].URL == f.URL {
+			config.Unlock()
+			return false
+		}
+	}
+
+	config.Filters = append(config.Filters, f)
+	config.Unlock()
+	return true
+}
+
 // Load filters from the disk
 // And if any filter has zero ID, assign a new one
 func loadFilters() {