diff --git a/src/http_server/new_york_transit_line/parser.cpp b/src/http_server/new_york_transit_line/parser.cpp
index d7c3460..69e4a78 100644
--- a/src/http_server/new_york_transit_line/parser.cpp
+++ b/src/http_server/new_york_transit_line/parser.cpp
@@ -542,6 +542,7 @@ namespace nytl {
                 std::string argname = readName(ctx);
                 ASSERT(!argname.empty(), "Expected argument name");
                 if (argname != "_") {
+                    ASSERT(arglist.count(argname) == 0, "Repeated argument (" + argname + ")");
                     size_t k = arglist.size();
                     arglist[argname] = k;
                 }