Index: openacs-4/packages/proctoring-support/lib/proctoring-upload.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/proctoring-support/lib/Attic/proctoring-upload.tcl,v
diff -u -N -r1.1.2.4 -r1.1.2.5
--- openacs-4/packages/proctoring-support/lib/proctoring-upload.tcl	1 Sep 2020 11:34:53 -0000	1.1.2.4
+++ openacs-4/packages/proctoring-support/lib/proctoring-upload.tcl	6 Apr 2021 14:10:55 -0000	1.1.2.5
@@ -53,8 +53,24 @@
     ad_script_abort
 }
 
+# A client-side timeout might still end up being processed by the
+# server. Here we make sure we do not process pictures twice for a
+# specific user.
+if {[::proctoring::picture_already_received_p \
+         -object_id $object_id \
+         -user_id $user_id \
+         -file ${file.tmpfile}]} {
+    # We do not tell anything to the client: for what they are
+    # concerned, file has been received and they should go on with
+    # their lives.
+    ns_log warning "Proctoring: user $user_id tried to upload content twice, skipping silently..."
+    ns_return 200 text/plain OK
+    ad_script_abort
+}
+
 set timestamp [clock seconds]
 set file_path $proctoring_dir/${name}-${type}-$timestamp.$extension
+
 file mkdir -- $proctoring_dir
 file rename -force -- ${file.tmpfile} $file_path
 
Index: openacs-4/packages/proctoring-support/tcl/proctoring-procs.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/proctoring-support/tcl/Attic/proctoring-procs.tcl,v
diff -u -N -r1.1.2.8 -r1.1.2.9
--- openacs-4/packages/proctoring-support/tcl/proctoring-procs.tcl	16 Mar 2021 17:11:22 -0000	1.1.2.8
+++ openacs-4/packages/proctoring-support/tcl/proctoring-procs.tcl	6 Apr 2021 14:10:55 -0000	1.1.2.9
@@ -180,3 +180,36 @@
           and (end_time is null or end_time >= cast(current_timestamp as time))
     }]
 }
+
+ad_proc -private ::proctoring::picture_already_received_p {
+    -object_id:required
+    -user_id:required
+    -file:required
+} {
+    Returns if the picture is the same the users sent the last time
+    they uploaded a proctoring file.
+
+    @param object_id id of the proctored object
+    @param user_id id of the proctored user
+    @param file an absolute path to a file in the filesystem
+
+    @return boolean
+} {
+    set cache_name proctoring_checksums_cache
+
+    # Make sure the checksum of current file is not the same as the
+    # one we have in the cache.
+    set checksum [ns_md file -digest sha1 $file]
+    set already_received_p false
+    if {[ns_cache_get $cache_name ${object_id}_${user_id} cached_checksum]} {
+        if {$checksum eq $cached_checksum} {
+            set already_received_p true
+        }
+    }
+
+    # Update in any case the cache to renew the expiration
+    ns_cache_eval -force -- $cache_name ${object_id}_${user_id} set v $checksum
+
+    return $already_received_p
+}
+
Index: openacs-4/packages/proctoring-support/tcl/proctoring-support-init.tcl
===================================================================
RCS file: /usr/local/cvsroot/openacs-4/packages/proctoring-support/tcl/Attic/proctoring-support-init.tcl,v
diff -u -N
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ openacs-4/packages/proctoring-support/tcl/proctoring-support-init.tcl	6 Apr 2021 14:10:56 -0000	1.1.2.1
@@ -0,0 +1,19 @@
+ad_library {
+    Startup script for proctoring-support
+}
+
+# This cache will store the checksum of the last picture that has been
+# received on behalf of a user while proctoring a specific object.
+# This is needed to implement a server-side check that detects
+# pictures being sent multiple times, e.g. when a request resulting in
+# a client timeout still gets processed by the server.
+
+set cache_name proctoring_checksums_cache
+set cache_size 1MB ; # ~ 25000 40bytes sha1 entries
+set cache_timeout 1ms
+set cache_expires [expr {2 * 60 * 60}] ; # 2 hours
+
+ns_cache_create \
+    -timeout $cache_timeout \
+    -expires $cache_expires -- $cache_name $cache_size
+