Add custom ui regression test for duplicated blocks calls

testing/tests/custom_ui/duplicated_block_calls.rs

@@ -0,0 +1,30 @@
+use askama::Template;
+
+// Check if the block call is duplicated in the current template.
+#[derive(Template)]
+#[template(
+    source = r##"{% extends "base.html" %}
+
+{% block content %}{% endblock %}
+{% block content %}{% endblock %}
+"##,
+    ext = "txt",
+)]
+struct X {
+    title: &'static str,
+}
+
+// Check if the block call is called in the extended template and in the current one.
+#[derive(Template)]
+#[template(
+    source = r##"{% extends "child.html" %}
+
+{% block content %}another{% endblock %}
+"##,
+    ext = "txt",
+)]
+struct X2 {
+    title: &'static str,
+}
+
+fn main() {}

testing/tests/custom_ui/duplicated_block_calls.stderr

@@ -0,0 +1,2 @@
+⚠️ X.txt:4:3: block `content` was already called at `X.txt:3:3` so the previous one will be ignored
+⚠️ X2.txt:3:3: block `content` was already called at `testing/templates/child.html:2:3` so the previous one will be ignored