Check for identical templates before saving

ftl/core/card-templates.ftl

@@ -21,6 +21,7 @@ card-templates-night-mode = Night Mode
 card-templates-add-mobile-class = Add Mobile Class
 card-templates-preview-settings = Options
 card-templates-invalid-template-number = Card template { $number } in notetype '{ $notetype }' has a problem.
+card-templates-identical-front = Its front side is identical with the one of card template { $number }.
 card-templates-see-preview = See the render preview for more information.
 card-templates-changes-saved = Changes saved.
 card-templates-discard-changes = Discard changes?

rslib/src/error/mod.rs

@@ -66,6 +66,7 @@ impl AnkiError {
                     tr.card_templates_invalid_template_number(err.ordinal + 1, &err.notetype);
                 let details = match err.details {
                     TemplateSaveErrorDetails::TemplateError => tr.card_templates_see_preview(),
+                    TemplateSaveErrorDetails::Duplicate(i) => tr.card_templates_identical_front(i),
                 };
                 format!("{}<br>{}", header, details)
             }
@@ -145,5 +146,6 @@ pub struct TemplateSaveError {
 
 #[derive(Debug, PartialEq)]
 pub enum TemplateSaveErrorDetails {
+    Duplicate(usize),
     TemplateError,
 }

rslib/src/notetype/mod.rs

@@ -272,6 +272,21 @@ impl Notetype {
             });
     }
 
+    fn ensure_template_fronts_unique(&self) -> Result<()> {
+        for i in 1..self.templates.len() {
+            for j in 0..i {
+                if self.templates[i].config.q_format == self.templates[j].config.q_format {
+                    return Err(AnkiError::TemplateSaveError(TemplateSaveError {
+                        notetype: self.name.clone(),
+                        ordinal: i,
+                        details: TemplateSaveErrorDetails::Duplicate(j + 1),
+                    }));
+                }
+            }
+        }
+        Ok(())
+    }
+
     pub(crate) fn normalize_names(&mut self) {
         ensure_string_in_nfc(&mut self.name);
         for f in &mut self.fields {
@@ -314,6 +329,7 @@ impl Notetype {
         self.fix_template_names()?;
         self.ensure_names_unique();
         self.reposition_sort_idx();
+        self.ensure_template_fronts_unique()?;
 
         let parsed_templates = self.parsed_templates();
         let invalid_card_idx = parsed_templates