diff --git a/bin/renumber b/bin/renumber
index 72db1a5..f7b7a61 100755
--- a/bin/renumber
+++ b/bin/renumber
@@ -9,6 +9,13 @@ end
 
 content = File.read(filename)
 
+refs = content.scan(/\[\d+\]:/)
+
+unless refs == refs.uniq
+  warn "Error: duplicate refs detected"
+  exit
+end
+
 links = content.scan(/\[\d+\]/).uniq
 
 links.zip(1..).each do |old_id, new_id|