head.headers_mut()
     .insert(header::CONTENT_ENCODING, encoding.to_header_value());
+// Content length may no longer be valid once encoding is applied
+head.headers_mut().remove(header::CONTENT_LENGTH);

 let mut vary_values: Vec<String> = head
     .headers()
     .get_all(header::VARY)
     .filter_map(|v| v.to_str().ok())
     .flat_map(|s| s.split(','))
     .map(|v| v.trim())
     .filter(|v| !v.is_empty())
     .map(str::to_string)
     .collect();
 
-// Add accept-encoding if not already present
-if !vary_values
-    .iter()
-    .any(|v| v.eq_ignore_ascii_case("accept-encoding"))
-{
+// Ensure "accept-encoding" is present (case-insensitive)
+if !vary_values.iter().any(|v| v.eq_ignore_ascii_case("accept-encoding")) {
     vary_values.push("accept-encoding".to_string());
 }
 
-// Remove all existing Vary headers and insert a single combined one
-head.headers_mut().remove(header::VARY);
-let combined = vary_values.join(", ");
-if let Ok(header_value) = HeaderValue::from_str(&combined) {
+// Deduplicate tokens case-insensitively while preserving original casing of first occurrence
+let mut seen = std::collections::HashSet::new();
+let mut deduped: Vec<String> = Vec::new();
+for v in vary_values.into_iter() {
+    let key = v.to_ascii_lowercase();
+    if seen.insert(key) {
+        deduped.push(v);
+    }
+}
+
+// Build combined header and only replace if valid
+let combined = deduped.join(", ");
+if HeaderValue::from_str(&combined).is_ok() {
+    head.headers_mut().remove(header::VARY);
+    // safe to unwrap because checked above
+    let header_value = HeaderValue::from_str(&combined).unwrap();
     head.headers_mut().insert(header::VARY, header_value);
 }

+let combined = vary_values
+    .into_iter()
+    .map(|s| s.trim().to_string())
+    .filter(|s| !s.is_empty())
+    .collect::<Vec<_>>()
+    .join(", ");
 if let Ok(header_value) = HeaderValue::from_str(&combined) {
     head.headers_mut().insert(header::VARY, header_value);
 }