diff --git a/src/ParsingDriver.cc b/src/ParsingDriver.cc
index c3fc77f49f2567fc3330436bab3bc4b0e54c7f4d..c615eb48c25766e1c4bd8820b33ec73b6e348485 100644
--- a/src/ParsingDriver.cc
+++ b/src/ParsingDriver.cc
@@ -279,6 +279,9 @@ ParsingDriver::add_predetermined_variable(const string &name)
 void
 ParsingDriver::add_equation_tags(string key, string value)
 {
+  if (eq_tags.find(key) != eq_tags.end())
+    error("Tag '" + key + "' cannot be declared twice for the same equation");
+
   eq_tags[key] = value;
 
   transform(key.begin(), key.end(), key.begin(), ::tolower);