I was wrong, html escape filter is appended even if an html escape filter is already there.

js/test_cases/filters/README.md

@@ -4,5 +4,5 @@ HTML Escaping
 Dust automatically applies the `|h` filter to html escape unless `|s` is applied to disable automatic html escaping. It seems that if you manually specify `|h` and `|s` in the same filter, then it still html escapes, so my theory on the logic is:
 
 Iterate over all filters  
-If `|s` or `|h` is not present append `|h`, otherwise, leave filters as-in  
+If `|s` is not present append `|h`, otherwise, leave filters as-in  
 During render, `|s` does nothing, so we can just remove it on the dust side to prevent confusion.  

js/test_cases/filters/main.dust

@@ -2,6 +2,7 @@ Special characters: {special_characters}{~n}
 Special characters html escaping disabled: {special_characters|s}{~n}
 Special characters html escaping disabled and enabled: {special_characters|s|h}{~n}
 Special characters html escaping enabled and disabled: {special_characters|h|s}{~n}
+Special characters html escaped once: {special_characters|h}{~n}
 Special characters html escaped twice: {special_characters|h|h}{~n}
 
 

src/renderer/renderer.rs

@@ -510,10 +510,7 @@ impl<'a> DustRenderer<'a> {
 
         // If the user has not specified any escaping filter (|s or
         // |h), automatically add an html escape filter
-        if !filters
-            .iter()
-            .any(|f| f == &Filter::DisableHtmlEncode || f == &Filter::HtmlEncode)
-        {
+        if !filters.iter().any(|f| f == &Filter::DisableHtmlEncode) {
             final_filters.push(Filter::HtmlEncode);
         }
         final_filters