diff --git a/config.c b/config.c
index 27867b0..8ce8ad4 100644
--- a/config.c
+++ b/config.c
@@ -155,3 +155,29 @@ int config_read(char *name, struct config *cfg)
 	fclose(fp);
 	return 0;
 }
+
+/* returns the number matching that interface, or -1 on failure */
+int config_create_interface(char *name, struct config *cfg)
+{
+	struct interface *iface;
+	int i;
+
+	if (cfg->nports >= MAX_PORTS) {
+		return -1;
+	}
+
+	iface = &cfg->iface[cfg->nports];
+
+	/* only create each interface once (by name) */
+	for(i = 0; i < cfg->nports; i++) {
+		if (0 == strncmp(name, cfg->iface[i].name, MAX_IFNAME_SIZE))
+			return i;
+	}
+
+	strncpy(iface->name, name, MAX_IFNAME_SIZE);
+	iface->dm = cfg->dm;
+	iface->transport = cfg->transport;
+	cfg->nports++;
+
+	return i;
+}
diff --git a/config.h b/config.h
index 306fd48..cdade16 100644
--- a/config.h
+++ b/config.h
@@ -55,5 +55,6 @@ struct config {
 };
 
 int config_read(char *name, struct config *cfg);
+int config_create_interface(char *name, struct config *cfg);
 
 #endif
diff --git a/ptp4l.c b/ptp4l.c
index 8c47ee1..7aa872a 100644
--- a/ptp4l.c
+++ b/ptp4l.c
@@ -151,12 +151,7 @@ int main(int argc, char *argv[])
 			config = optarg;
 			break;
 		case 'i':
-			if (*nports < MAX_PORTS) {
-				strncpy(iface[*nports].name, optarg, MAX_IFNAME_SIZE);
-				iface[*nports].dm = *dm;
-				iface[*nports].transport = *transport;
-				(*nports)++;
-			} else {
+			if (config_create_interface(optarg, &cfg_settings) < 0) {
 				fprintf(stderr, "too many interfaces\n");
 				return -1;
 			}