diff --git a/prism/src/parser/ast/ModulesFile.java b/prism/src/parser/ast/ModulesFile.java
index caf589a2..3048a8d6 100644
--- a/prism/src/parser/ast/ModulesFile.java
+++ b/prism/src/parser/ast/ModulesFile.java
@@ -403,6 +403,12 @@ public class ModulesFile extends ASTElement
 	 */
 	public void tidyUp() throws PrismLangException
 	{
+		// Clear lists that will generated by this method 
+		// (in case it has already been called previously).
+		formulaIdents.clear();
+		constantIdents.clear();
+		varIdents.clear();
+		
 		// Expansion of formulas and renaming
 
 		// Check formula identifiers