The following patch seems to do the trick after some _very_ limited testing.

diff -u ./grml-chroot /usr/sbin/grml-chroot
--- ./grml-chroot       2009-04-04 07:31:55.614147651 +0000
+++ /usr/sbin/grml-chroot       2009-04-04 07:30:55.290805628 +0000
@@ -9,6 +9,7 @@
 PROG_NAME_=$(basename $0)
 DEST_=""
 MOUNTED_=""     # all mounted destinations
+STATEDIR="tmp/grml-chroot"

 function die
 {
@@ -26,6 +27,16 @@
 EOT
 }

+function isMounted
+{
+    local dir="$1"
+    if < /proc/mounts cut -d\  -f 2 | grep -q "$dir"; then
+        return 0
+    else
+        return 1
+    fi
+}
+
 function storeMounts
 {
     local to_append_="$1"
@@ -44,12 +55,14 @@

     local all_options_=""

-    if [[ $options_ == "--bind" ]]; then
-        all_options_="--bind $type_"
-    else
-        all_options_="-t $type_ none"
+    if ! isMounted ${DEST_}/$dest_; then
+        if [[ $options_ == "--bind" ]]; then
+            all_options_="--bind $type_"
+        else
+            all_options_="-t $type_ none"
+        fi
+        mount $all_options_ "${DEST_}/$dest_" && storeMounts "$dest_"
     fi
-    mount $all_options_ "${DEST_}/$dest_" && storeMounts "$dest_"
 }

 function umount_all
@@ -87,9 +100,14 @@
 mountit "proc"  "proc"
 mountit "sysfs" "sys"
 mountit "/dev"   "dev"   "--bind"
+mkdir -p "$DEST_/$STATEDIR"
+touch "$DEST_/$STATEDIR/$$"
 if (( $# < 1 )); then
     chroot "$DEST_"
 else
     chroot "$DEST_" "$@"
 fi
-umount_all
+rm "$DEST_/$STATEDIR/$$"
+if rmdir $DEST_/$STATEDIR 2>/dev/null; then
+  umount_all
+fi