diff --git a/lua/nvim-treesitter/configs.lua b/lua/nvim-treesitter/configs.lua
index e64dc28e4..6be0c0148 100644
--- a/lua/nvim-treesitter/configs.lua
+++ b/lua/nvim-treesitter/configs.lua
@@ -405,6 +405,18 @@ function M.is_enabled(mod, lang, bufnr)
   return true
 end
 
+---Combine two list-like tables without duplicates
+local union = function(x, y)
+  local map = {}
+  for _, v in ipairs(x) do
+    map[v] = true
+  end
+  for _, v in ipairs(y) do
+    map[v] = true
+  end
+  return vim.tbl_keys(map)
+end
+
 ---Setup call for users to override module configurations.
 ---@param user_data TSConfig module overrides
 function M.setup(user_data)
@@ -422,7 +434,7 @@ function M.setup(user_data)
 
   local ensure_installed = user_data.ensure_installed or config.ensure_installed
   if type(ensure_installed) == "table" and type(config.ensure_installed) == "table" then
-    vim.list_extend(ensure_installed, config.ensure_installed)
+    union(ensure_installed, config.ensure_installed)
   end
   if #ensure_installed > 0 then
     if user_data.sync_install then