diff --git a/tools/ci-build b/tools/ci-build
index 89c4cb8d3..52d22ee80 100755
--- a/tools/ci-build
+++ b/tools/ci-build
@@ -131,6 +131,9 @@ case "${OCAML_ARCH}" in
   *) arch_error;;
 esac
 
+# Make sure two builds won't use the same install directory
+instdir="$instdir-$$"
+
 #########################################################################
 # On Windows, cleanup processes that may remain from previous run