diff --git a/pkgs/stdenv/generic/setup.sh b/pkgs/stdenv/generic/setup.sh
index 7168ce4084c..a73474c952e 100644
--- a/pkgs/stdenv/generic/setup.sh
+++ b/pkgs/stdenv/generic/setup.sh
@@ -157,7 +157,8 @@ addToSearchPathWithCustomDelimiter() {
     local delimiter="$1"
     local varName="$2"
     local dir="$3"
-    if [ -d "$dir" ]; then
+    if [[ -d "$dir" && "${!varName:+${delimiter}${!varName}${delimiter}}" \
+          != *"${delimiter}${dir}${delimiter}"* ]]; then
         export "${varName}=${!varName:+${!varName}${delimiter}}${dir}"
     fi
 }