+ * Linked list of duplicate blocks.
+ *
+ * The list is composed of two parts. The first part of the
+ * list (from duplist through the node pointed to by muldup)
+ * contains a single copy of each duplicate block that has been
+ * found. The second part of the list (from muldup to the end)
+ * contains duplicate blocks that have been found more than once.
+ * To check if a block has been found as a duplicate it is only
+ * necessary to search from duplist through muldup. To find the
+ * total number of times that a block has been found as a duplicate
+ * the entire list must be searched for occurences of the block
+ * in question. The following diagram shows a sample list where
+ * w (found twice), x (found once), y (found three times), and z
+ * (found once) are duplicate block numbers:
+ *
+ * w -> y -> x -> z -> y -> w -> y
+ * ^ ^
+ * | |
+ * duplist muldup