diff --git a/src/cuirass/database.scm b/src/cuirass/database.scm
index 91133c2..9c7e69a 100644
--- a/src/cuirass/database.scm
+++ b/src/cuirass/database.scm
@@ -181,15 +181,30 @@ string."
             ((char=? char #\')  (loop (cons* char char chars)))
             (else (loop (cons char chars)))))))
 
+;; Extended error codes (see <sqlite3.h>).
+;; XXX: This should be defined by (sqlite3).
+(define SQLITE_CONSTRAINT 19)
+(define SQLITE_CONSTRAINT_PRIMARYKEY
+  (logior SQLITE_CONSTRAINT (ash 6 8)))
+
 (define (db-add-build db build)
-  "Store BUILD in database DB."
-  (sqlite-exec db "\
+  "Store BUILD in database DB.  This is idempotent."
+  (catch 'sqlite-error
+    (lambda ()
+      (sqlite-exec db "\
 INSERT INTO Builds (derivation, evaluation, log, output)\
   VALUES ('~A', '~A', '~A', '~A');"
-               (assq-ref build #:derivation)
-               (assq-ref build #:eval-id)
-               (assq-ref build #:log)
-               (assq-ref build #:output))
+                   (assq-ref build #:derivation)
+                   (assq-ref build #:eval-id)
+                   (assq-ref build #:log)
+                   (assq-ref build #:output)))
+    (lambda (key who code . rest)
+      ;; If we get a primary-key-constraint-violated error, that means we have
+      ;; already inserted the same (derivation,eval-id,log) tuple, which we
+      ;; can safely ignore.
+      (unless (= code SQLITE_CONSTRAINT_PRIMARYKEY)
+        (apply throw key who code rest))))
+
   (last-insert-rowid db))
 
 (define (db-get-stamp db spec)