public String toString()
        {
            return builder.toString();
        }
This combinator adds duplicates to an arbitrary list. We prefer this to JQwik's built-in functionality,
as that is inefficient and dominates test runs.
<p>
This method works by generating a list of integers, which represent, in an alternating manner,
the number of items to skip before <i>selecting</i> an item to duplicate
and the number of items to skip before <i>inserting</i> the duplicate.

@param maxDuplicates the maximum number of duplicates to add
@param arbitrary     the arbitrary list to duplicate items in
@param <T>           the type of the list
@return an arbitrary list with duplicates
sbe-tool/src/propertyTest/java/uk/co/real_logic/sbe/properties/arbitraries/SbeArbitraries.java