improve error when srcs is used with directories with the same post-hash name

pkgs/stdenv/generic/setup.sh

@@ -992,13 +992,22 @@ stripHash() {
 unpackCmdHooks+=(_defaultUnpack)
 _defaultUnpack() {
     local fn="$1"
+    local destination
 
     if [ -d "$fn" ]; then
 
+        destination="$(stripHash "$fn")"
+
+        if [ -e "$destination" ]; then
+            echo "Cannot copy $fn to $destination: destination already exists!"
+            echo "Did you specify two \"srcs\" with the same \"name\"?"
+            return 1
+        fi
+
         # We can't preserve hardlinks because they may have been
         # introduced by store optimization, which might break things
         # in the build.
-        cp -pr --reflink=auto -- "$fn" "$(stripHash "$fn")"
+        cp -pr --reflink=auto -- "$fn" "$destination"
 
     else