diff --git a/prism/src/parser/ast/PropertiesFile.java b/prism/src/parser/ast/PropertiesFile.java
index 68b5167d..3acd6ae2 100644
--- a/prism/src/parser/ast/PropertiesFile.java
+++ b/prism/src/parser/ast/PropertiesFile.java
@@ -182,6 +182,10 @@ public class PropertiesFile extends ASTElement
 	
 	public void tidyUp() throws PrismLangException
 	{
+		// Clear lists that will generated by this method 
+		// (in case it has already been called previously).
+		allIdentsUsed.clear();
+		
 		// Check formula identifiers
 		checkFormulaIdents();
 		// Find all instances of formulas (i.e. locate idents which are formulas),