diff --git a/checkin_notes b/checkin_notes
index 5f01125b20..8902f037f0 100755
--- a/checkin_notes
+++ b/checkin_notes
@@ -27044,6 +27044,13 @@ Bruce 11 April 2005
       sleep, and then try again, but DON'T find more work available, give up
       trying to make work for that file.
 
+    - File upload handler: to prevent multiple instances of file_upload_handler
+      from trying to upload the SAME file, use lockf() to place an advisory lock on
+      the file.  David, I probably should have discussed this with you first, but it's
+      too early in the morning.  Please revert if this is a mistake!
+
     sched/
         sched_locality.C
+        file_upload_handler.C
+
 
diff --git a/sched/file_upload_handler.C b/sched/file_upload_handler.C
index ff9b166c2a..480aa062f9 100644
--- a/sched/file_upload_handler.C
+++ b/sched/file_upload_handler.C
@@ -142,13 +142,26 @@ int copy_socket_to_file(FILE* in, char* path, double offset, double nbytes) {
     unsigned char buf[BLOCK_SIZE];
     char buf2[256];
     FILE* out;
-    int retval, n, m;
+    int retval, n, m, fd, lockret;
 
     out = fopen(path, "ab");
     if (!out) {
         return return_error(ERR_TRANSIENT, "can't open file %s: %s", path, strerror(errno));
     }
 
+    // get file descriptor for locking purposes
+    fd=fileno(out);
+    if (fd<0) {
+        return return_error(ERR_TRANSIENT, "can't get file descriptor for file %s: %s", path, strerror(errno));
+    }
+
+    // Put an advisory lock on the file.  This will prevent OTHER instances of file_upload_handler
+    // from being able to write to the file.
+    lockret=lockf(fd, F_TLOCK, 0);
+    if (lockret) {
+        return return_error(ERR_TRANSIENT, "can't get exclusive lock on file %s: %s", path, strerror(errno));
+    }
+
     // TODO: use a 64-bit variant
 
     retval = fseek(out, (long)offset, SEEK_CUR);
@@ -424,8 +437,8 @@ int pid;
 
 void boinc_catch_signal(int signal_num) {
     log_messages.printf(SCHED_MSG_LOG::CRITICAL,
-        "FILE=%s (%.0f bytes left) IP=%s PID=%d caught signal %d [%s]\n",
-        this_filename, bytes_left, get_remote_addr(), pid, signal_num, sys_siglist[signal_num]
+        "PID=%d FILE=%s (%.0f bytes left) IP=%s caught signal %d [%s]\n",
+        pid, this_filename, bytes_left, get_remote_addr(), signal_num, sys_siglist[signal_num]
     );
     exit(1);
 }