diff --git a/src/fe-common/core/module-formats.c b/src/fe-common/core/module-formats.c
index e98889a4..ba6bbbe6 100644
--- a/src/fe-common/core/module-formats.c
+++ b/src/fe-common/core/module-formats.c
@@ -37,6 +37,7 @@ FORMAT_REC fecommon_core_formats[] = {
 	{ "error_server_sticky", "Window's server is sticky and it cannot be changed without -unsticky option", 0 },
 	{ "set_server_sticky", "Window's server set sticky", 1, { 0 } },
 	{ "unset_server_sticky", "Window's server isn't sticky anymore", 0 },
+	{ "window_name_not_unique", "Window names must be unique", 1, { 0 } },
 	{ "window_level", "Window level is now $0", 1, { 0 } },
 	{ "windowlist_header", "Ref Name                 Active item     Server          Level", 0 },
 	{ "windowlist_line", "$[3]0 %|$[20]1 $[15]2 $[15]3 $4", 5, { 1, 0, 0, 0, 0 } },
diff --git a/src/fe-common/core/module-formats.h b/src/fe-common/core/module-formats.h
index 2b25530f..e5e7d99b 100644
--- a/src/fe-common/core/module-formats.h
+++ b/src/fe-common/core/module-formats.h
@@ -14,7 +14,8 @@ enum {
 	TXT_REFNUM_TOO_LOW,
         TXT_ERROR_SERVER_STICKY,
         TXT_SET_SERVER_STICKY,
-        TXT_UNSET_SERVER_STICKY,
+	TXT_UNSET_SERVER_STICKY,
+        TXT_WINDOW_NAME_NOT_UNIQUE,
         TXT_WINDOW_LEVEL,
 	TXT_WINDOWLIST_HEADER,
 	TXT_WINDOWLIST_LINE,
diff --git a/src/fe-common/core/window-commands.c b/src/fe-common/core/window-commands.c
index 8ba83ea4..34cd3099 100644
--- a/src/fe-common/core/window-commands.c
+++ b/src/fe-common/core/window-commands.c
@@ -460,7 +460,12 @@ static void cmd_window_number(const char *data)
 /* SYNTAX: WINDOW NAME <name> */
 static void cmd_window_name(const char *data)
 {
-        window_set_name(active_win, data);
+	if (window_find_name(data) == NULL)
+		window_set_name(active_win, data);
+	else {
+		printformat_window(active_win, MSGLEVEL_CLIENTERROR,
+				   TXT_WINDOW_NAME_NOT_UNIQUE, data);
+	}
 }
 
 /* we're moving the first window to last - move the first contiguous block