diff -ur b/atd.c a/atd.c
--- b/atd.c	2021-04-25 03:31:30.000000000 +0200
+++ a/atd.c	2021-05-21 13:00:45.135020670 +0200
@@ -74,6 +74,9 @@
 #include <syslog.h>
 #endif
 
+#include <sys/file.h>
+#include <utime.h>
+
 /* Local headers */
 
 #include "privs.h"
@@ -275,7 +278,7 @@
  * mail to the user.
  */
     pid_t pid;
-    int fd_out, fd_in;
+    int fd_out, fd_in, fd_std;
     char jobbuf[9];
     char *mailname = NULL;
     int mailsize = 128;
@@ -390,6 +393,10 @@
 
     fcntl(fd_in, F_SETFD, fflags & ~FD_CLOEXEC);
 
+    if (flock(fd_in, LOCK_EX | LOCK_NB) != 0)
+	    perr("Somebody already locked the job %8lu (%.500s) - "
+	     "aborting", jobno, filename);
+
     /*
      * If the spool directory is mounted via NFS `atd' isn't able to
      * read from the job file and will bump out here.  The file is
@@ -520,10 +527,7 @@
 	PRIV_END
     }
     /* We're the parent.  Let's wait.
-     */
-    close(fd_in);
-
-    /* We inherited the master's SIGCHLD handler, which does a
+       We inherited the master's SIGCHLD handler, which does a
        non-blocking waitpid. So this blocking one will eventually
        return with an ECHILD error. 
      */
@@ -548,14 +552,14 @@
     /* some sendmail implementations are confused if stdout, stderr are
      * not available, so let them point to /dev/null
      */
-    if ((fd_in = open("/dev/null", O_WRONLY)) < 0)
+    if ((fd_std = open("/dev/null", O_WRONLY)) < 0)
 	perr("Could not open /dev/null.");
-    if (dup2(fd_in, STDOUT_FILENO) < 0)
+    if (dup2(fd_std, STDOUT_FILENO) < 0)
 	perr("Could not use /dev/null as standard output.");
-    if (dup2(fd_in, STDERR_FILENO) < 0)
+    if (dup2(fd_std, STDERR_FILENO) < 0)
 	perr("Could not use /dev/null as standard error.");
-    if (fd_in != STDOUT_FILENO && fd_in != STDERR_FILENO)
-	close(fd_in);
+    if (fd_std != STDOUT_FILENO && fd_std != STDERR_FILENO)
+	close(fd_std);
 
     if (unlink(filename) == -1)
         syslog(LOG_WARNING, "Warning: removing output file for job %li failed: %s",
@@ -563,7 +567,12 @@
 
     /* The job is now finished.  We can delete its input file.
      */
-    chdir(ATJOB_DIR);
+    if (chdir(ATJOB_DIR) != 0)
+	perr("Somebody removed %s directory from under us.", ATJOB_DIR);
+
+    /* This also removes the flock */
+    (void)close(fd_in);
+
     unlink(newname);
     free(newname);
 
@@ -673,16 +682,18 @@
 
 	/* Skip lock files */
 	if (queue == '=') {
-            /* FIXME: calhariz */
-            /* I think the following code is broken, but commenting it
-               may cause unknow side effects.  Make a release and see
-               in the wild how it works. For more information see:
-               https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=818508 */
-
-	    /* if ((buf.st_nlink == 1) && (run_time + CHECK_INTERVAL <= now)) { */
-	    /*     /\* Remove stale lockfile FIXME: lock the lockfile, if you fail, it's still in use. *\/ */
-	    /*     unlink(dirent->d_name); */
-	    /* } */
+	    if ((buf.st_nlink == 1) && (run_time + CHECK_INTERVAL <= now)) { 
+		    int fd;
+
+            fd = open(dirent->d_name, O_RDONLY);
+            if (fd != -1) {
+                if (flock(fd, LOCK_EX | LOCK_NB) == 0) {
+                    unlink(dirent->d_name);
+                    syslog(LOG_NOTICE, "removing stale lock file %s\n", dirent->d_name);
+                }
+                (void)close(fd);
+            }
+	    }
 	    continue;
 	}
 	/* Skip any other file types which may have been invented in