diff --git a/Modules/FindX11.cmake b/Modules/FindX11.cmake
index 0b63bc9bd..01ba478ad 100644
--- a/Modules/FindX11.cmake
+++ b/Modules/FindX11.cmake
@@ -324,6 +324,9 @@ if (UNIX)
      set(X11_SM_FOUND TRUE)
   endif()
 
+  # Most of the X11 headers will be in the same directories, avoid
+  # creating a huge list of duplicates.
+  list(REMOVE_DUPLICATES X11_INCLUDE_DIR)
 
   # Deprecated variable for backwards compatibility with CMake 1.4
   if (X11_X11_INCLUDE_PATH AND X11_LIBRARIES)