diff --git a/README.md b/README.md
index 3c45b60..627c9f1 100644
--- a/README.md
+++ b/README.md
@@ -65,6 +65,7 @@ Firestarter, in addition to spawning the programs in the default configs, also i
 | 52 | Failed to create configuration directory |
 | 53 | Failed to create logging directory |
 | 54 | `HOME` does not exist or is unreadable |
+| 55 | Firestarter is already running |
 | 70 | No configuration files available |
 
 ## Idiosyncracies
diff --git a/firestarter b/firestarter
index 7530b39..b7799cf 100755
--- a/firestarter
+++ b/firestarter
@@ -150,6 +150,8 @@ step_preexecute() {
 	[ -n "$_optdryrun" ] && return 0
 	[ -r "$HOME/.xsessionrc" ] && . "$HOME/.xsessionrc"
 	export XDG_CURRENT_DESKTOP="${XDG_CURRENT_DESKTOP:-firestarter}"
+	[ -n "$FIRESTARTER" ] && error "Firestarter is already running: $FIRESTARTER" 55
+	export FIRESTARTER="$BASHPID"
 	# dbus
 	if \
 		[ -z "$DBUS_SESSION_BUS_ADDRESS" ] && \