diff --git a/tests/LightyTest.pm b/tests/LightyTest.pm
index e38de06d..0d31345a 100755
--- a/tests/LightyTest.pm
+++ b/tests/LightyTest.pm
@@ -152,6 +152,12 @@ sub handle_http {
 				if ($line =~ /^([^:]+):\s*(.+)$/) {
 					(my $h = $1) =~ tr/[A-Z]/[a-z]/;
 
+					if (defined $resp_hdr{$h}) {
+						diag(sprintf("header %s is duplicated: %s and %s\n",
+						             $h, $resp_hdr{$h}, $2));
+						return -1;	
+					}
+
 					$resp_hdr{$h} = $2;
 				} else {
 					return -1;