  Document listing on string collections is the task of finding all documents
where a pattern appears. It is regarded as the most fundamental document
retrieval problem, and is useful in various applications. Many of the
fastest-growing string collections are composed of very similar documents, such
as versioned code and document collections, genome repositories, etc. Plain
pattern-matching indexes designed for repetitive text collections achieve
orders-of-magnitude reductions in space. Instead, there are not many analogous
indexes for document retrieval. In this paper we present a simple document
listing index for repetitive string collections of total length $n$ that lists
the $ndoc$ distinct documents where a pattern of length $m$ appears in time
$\mathcal{O}(m+ndoc \cdot \log n)$. We exploit the repetitiveness of the
document array (i.e., the suffix array coarsened to document identifiers) to
grammar-compress it while precomputing the answers to nonterminals, and store
them in grammar-compressed form as well. Our experimental results show that our
index sharply outperforms existing alternatives in the space/time tradeoff map.
