--- esu.py-	2016-09-27 00:00:00.000000000 +0900
+++ esu.py	2016-09-28 00:00:00.000000000 +0900
@@ -685,7 +685,13 @@ def do_sdef(e, info):
 	# [ 'struct', name, body ]
 	(name, body) = e[1:3]
 	if name and body:
-		new_val( sdef_name(name), body, info )
+		name = sdef_name(name)
+		env = get_call_inf(info).get('env')
+		if name not in env:
+			new_val(name, body, info)
+			return
+		if env.get(name) != body:
+			err_exit("already other body exist '{}'".format(name))
 
 def do_expr(expr, info):
 	if info.get('verb') >= 2: