diff --git a/src/io/mandoline/impl.clj b/src/io/mandoline/impl.clj
index 89ce350..a35e06a 100644
--- a/src/io/mandoline/impl.clj
+++ b/src/io/mandoline/impl.clj
@@ -220,7 +220,7 @@
         (blank-slab metadata var-name dtype chunk-slice)))))
 
 (defn- update-chunk!
-  [index parent-index version-id store coordinate slab]
+  [index parent-index version-id store coordinate slab written-chunks]
   (loop [my-current-hash (chunk-at index coordinate version-id)]
     (let [bc (get-base-chunk my-current-hash index
                              parent-index store coordinate)
@@ -228,21 +228,26 @@
           hash (chunk/generate-id slab)
           ;; fixme implement ref-counting
           ref-count -1]
-      (->> slab
-           :data
-           .getDataAsByteBuffer
-           ;; TODO: The following line fixes a bug in ArrayChar, where
-           ;; .getDataAsByteBuffer doesn't rewind its position index. We
-           ;; should make sure that this doesn't accidentally rewind
-           ;; past the start of a chunk. We should also fix this
-           ;; upstream.
-           .rewind
-           ;; write or re-write chunk (generating new chunk ID)
-           (proto/write-chunk store hash ref-count))
+      ;; Write the chunk only if we need to (as far as we know).
+      (when-not (or (contains? @written-chunks hash)
+                    (= hash my-current-hash))
+        (->> slab
+             :data
+             .getDataAsByteBuffer
+             ;; TODO: The following line fixes a bug in ArrayChar, where
+             ;; .getDataAsByteBuffer doesn't rewind its position index. We
+             ;; should make sure that this doesn't accidentally rewind
+             ;; past the start of a chunk. We should also fix this
+             ;; upstream.
+             .rewind
+             ;; write or re-write chunk (generating new chunk ID)
+             (proto/write-chunk store hash ref-count))
+        (swap! written-chunks conj hash))
 
-      ;; If write-index returned nil, the transaction was aborted
-      ;; because another writer slipped in ahead of us. Re-merge this chunk.
-      (when-not (proto/write-index index coordinate my-current-hash hash)
+      (if (proto/write-index index coordinate my-current-hash hash)
+        hash
+        ;; If write-index returned nil, the transaction was aborted
+        ;; because another writer slipped in ahead of us. Re-merge this chunk.
         (let [sha1 (chunk-at index coordinate version-id)]
           (log/tracef (str "Retrying chunk update transaction at coordinate %s, "
                            "new sha1 %s") (pr-str coordinate) sha1)
@@ -261,10 +266,11 @@
   (let [{:keys [metadata var-name]} (proto/target index)
         {parent-metadata :metadata} (when parent-index
                                       (proto/target parent-index))
+        written-chunks (atom #{})
         update-fn (fn [slab coordinate]
                     (log/tracef "Updating chunk at %s" (pr-str coordinate))
                     (update-chunk! index parent-index (:version-id metadata)
-                                   store coordinate slab))]
+                                   store coordinate slab written-chunks))]
     (log/debugf "Writing to variable %s. Metadata: %s, parent %s"
                 var-name (pr-str metadata) (pr-str parent-metadata))
     (doseq [s slabs]
diff --git a/test/io/mandoline/impl_test.clj b/test/io/mandoline/impl_test.clj
new file mode 100644
index 0000000..98fd528
--- /dev/null
+++ b/test/io/mandoline/impl_test.clj
@@ -0,0 +1,35 @@
+(ns io.mandoline.impl-test (:require
+    [clojure.test :refer :all]
+    [io.mandoline
+     [chunk :as chunk]
+     [impl :as impl]
+     [slab :as slab]
+     [slice :as slice]]
+    [io.mandoline.impl.protocol :as proto]))
+
+
+;; Verify that when writing a variable, we don't repeatedly write an identical
+;; chunk. That's wasteful.
+(deftest write-variable-dedups-chunks
+  (let [chunks-written (atom 0)
+        float-type (slab/as-data-type Float/TYPE)
+        zeros-chunk (slab/empty float-type (slice/mk-slice [0 0] [10 10])) 
+        full-data (slab/empty
+                    float-type (slice/mk-slice [0 0] [1000 1000]) 1.0)]
+    (with-redefs [impl/get-base-chunk (constantly zeros-chunk)
+                  proto/write-chunk (fn [& _] (swap! chunks-written inc))
+                  proto/target (constantly
+                                 {:metadata
+                                  {:version-id :version-0
+                                   :variables {:var {:shape [:dim1 :dim2]}}
+                                   :dimensions {:dim1 1000
+                                                :dim2 1000}
+                                   :chunk-dimensions {:dim1 10
+                                                      :dim2 10}}
+                                  :var-name :var})
+                  proto/write-index (constantly true)
+                  impl/chunk-at (constantly (chunk/generate-id zeros-chunk))]
+      (impl/write-variable :store :index nil [full-data])
+      ;; Even though we were writing thousands of chunks, they were all the same,
+      ;; and hopefully we didn't write that one chunk too many times.
+      (is (< 0 @chunks-written 100)))))