diff --git a/crates/automation/src/a11y.rs b/crates/automation/src/a11y.rs
index b58b2d587b..81f79d14ee 100644
--- a/crates/automation/src/a11y.rs
+++ b/crates/automation/src/a11y.rs
@@ -3,7 +3,7 @@
 use pdfcraft_engine::a11y::{Category, Options, Report, Rule, Status};
 use serde_json::{Value, json};
 
-use crate::{Args, Automation, Result, ToolError, child, failed, write_atomic};
+use crate::{Args, Automation, Result, ToolError, child, failed, unused, write_atomic};
 
 fn status_id(s: Status) -> &'static str {
     match s {
@@ -347,6 +347,16 @@ impl Automation {
             out.push(match result {
                 Ok(r) => {
                     let target = child(&folder, &name);
+                    let target = if target.exists() {
+                        let stem =
+                            std::path::Path::new(&name).file_stem().map(|s| s.to_string_lossy().into_owned()).unwrap_or_else(|| "document".into());
+                        unused(&folder, &stem)
+                    } else {
+                        target
+                    };
+                    if target.exists() {
+                        return Err(failed(format!("{}: no unused output name available", target.display())));
+                    }
                     write_atomic(&target, &r.bytes)?;
                     json!({ "path": p, "output": target.to_string_lossy(), "log": r.log })
                 }
diff --git a/crates/automation/tests/automation.rs b/crates/automation/tests/automation.rs
index 6d0e8262f7..c509668474 100644
--- a/crates/automation/tests/automation.rs
+++ b/crates/automation/tests/automation.rs
@@ -3026,6 +3026,36 @@ fn actions_through_tools() {
     assert!(a.call("action_run", &json!({ "steps": [{ "step": "fly" }], "paths": ["a.pdf"], "folder": "x" })).is_err());
 }
 
+#[test]
+fn action_run_keeps_outputs_with_duplicate_input_basenames() {
+    let dir = workdir("action-duplicate-basenames");
+    std::fs::create_dir_all(dir.join("left")).unwrap();
+    std::fs::create_dir_all(dir.join("right")).unwrap();
+    std::fs::write(dir.join("left/report.pdf"), fixture(1)).unwrap();
+    std::fs::write(dir.join("right/report.pdf"), fixture(2)).unwrap();
+    let mut a = auto(&dir);
+    let args = json!({
+        "steps": [{ "step": "set_title", "arg": "Processed" }],
+        "paths": ["left/report.pdf", "right/report.pdf"],
+        "folder": "results"
+    });
+    let result = ok(&mut a, "action_run", args.clone());
+    let files = result["files"].as_array().unwrap();
+    assert_eq!(files.len(), 2);
+    let first_path = files[0]["output"].as_str().unwrap();
+    let second_path = files[1]["output"].as_str().unwrap();
+    assert!(first_path.ends_with("report.pdf"), "{first_path}");
+    assert!(second_path.ends_with("report (2).pdf"), "{second_path}");
+    let first_bytes = std::fs::read(first_path).unwrap();
+    assert_eq!(ok(&mut a, "doc_open", json!({ "path": first_path }))["pages"], 1);
+    assert_eq!(ok(&mut a, "doc_open", json!({ "path": second_path }))["pages"], 2);
+
+    let repeated = ok(&mut a, "action_run", args);
+    assert!(repeated["files"][0]["output"].as_str().unwrap().ends_with("report (3).pdf"));
+    assert!(repeated["files"][1]["output"].as_str().unwrap().ends_with("report (4).pdf"));
+    assert_eq!(std::fs::read(first_path).unwrap(), first_bytes, "an earlier output must not be replaced");
+}
+
 #[test]
 fn pdfa_through_tools() {
     let dir = workdir("pdfa");