diff --git a/b2/lang.y b/b2/lang.y
index 9cde6de..85d184a 100644
--- a/b2/lang.y
+++ b/b2/lang.y
@@ -590,6 +590,8 @@ provider:
 	WORD WORD float float TOK_NL
 		{
 			$$ = provider_add($1);
+			if ($$->curr)
+				yyerrorf("provider %s is already defined", $1);
 			$$->curr = currency_add($2);
 			$$->shipping = $3;
 			$$->minimum = $4;