diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 11185159c1..602f63fded 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -11,6 +11,10 @@
 typedef map<Path, PathSet> DrvRoots;
 typedef map<Path, Hash> DrvHashes;
 
+/* Cache for calls to addToStore(); maps source paths to the store
+   paths. */
+typedef map<Path, Path> SrcToStore;
+
 struct EvalState;
 
 /* Note: using a ATermVector is safe here, since when we call a primop
@@ -24,6 +28,7 @@ struct EvalState
     ATermMap primOps;
     DrvRoots drvRoots;
     DrvHashes drvHashes; /* normalised derivation hashes */
+    SrcToStore srcToStore; 
     Expr blackHole;
 
     unsigned int nrEvaluated;
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 31fff0b493..77721a5a8d 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -165,9 +165,15 @@ static void processBinding(EvalState & state, Expr e, Derivation & drv,
             if (isDerivation(srcPath))
                 throw Error(format("file names are not allowed to end in `%1%'")
                     % drvExtension);
-            Path dstPath(addToStore(srcPath));
-            printMsg(lvlChatty, format("copied source `%1%' -> `%2%'")
-                % srcPath % dstPath);
+            Path dstPath;
+            if (state.srcToStore[srcPath] != "")
+                dstPath = state.srcToStore[srcPath];
+            else {
+                dstPath = addToStore(srcPath);
+                state.srcToStore[srcPath] = dstPath;
+                printMsg(lvlChatty, format("copied source `%1%' -> `%2%'")
+                    % srcPath % dstPath);
+            }
             drv.inputSrcs.insert(dstPath);
             ss.push_back(dstPath);
         }