diff --git a/apps/deckcraft-cli/src/main.rs b/apps/deckcraft-cli/src/main.rs
index fd1d96ea..befd4476 100644
--- a/apps/deckcraft-cli/src/main.rs
+++ b/apps/deckcraft-cli/src/main.rs
@@ -200,6 +200,7 @@ fn render_transition(args: &[String]) -> Result<(), String> {
     if ts.is_empty() {
         return Err("render-transition needs --t".into());
     }
+    let paths = frame_paths(&out, &ts)?;
     let mut s = Session::new();
     open(&mut s, Some(&input), false)?;
     let st = s.doc().map_err(|e| e.to_string())?;
@@ -217,27 +218,39 @@ fn render_transition(args: &[String]) -> Result<(), String> {
         (prev.map(|p| render::render_slide(doc, p, &opts)), render::render_slide(doc, i, &opts), parts)
     });
     let aspect = sw / doc.slide_size.height.max(1.0);
-    for t in &ts {
+    for (k, t) in ts.iter().enumerate() {
         let img = match &cached {
             Some((old, new, parts)) => {
                 render::composite_transition(old.as_ref(), new, parts.as_ref(), &render::transition_frame(&tr.kind, &tr.option, *t, aspect))
             }
             None => render::render_transition(doc, i, *t, &opts),
         };
-        let path = if ts.len() == 1 {
-            out.clone()
-        } else {
-            let p = std::path::Path::new(&out);
-            let stem = p.file_stem().and_then(|x| x.to_str()).unwrap_or("frame");
-            let name = format!("{stem}-t{t:.3}.png");
-            p.with_file_name(name).to_string_lossy().into_owned()
-        };
-        std::fs::write(&path, img.to_png()).map_err(|e| format!("{path}: {e}"))?;
+        let path = &paths[k];
+        std::fs::write(path, img.to_png()).map_err(|e| format!("{path}: {e}"))?;
         outln!("{path} ({}×{}) {} {} t={t}", img.width, img.height, tr.kind, tr.option);
     }
     Ok(())
 }
 
+/// Output path of each frame. A single frame goes to OUT; several get `-t<value>` (three decimals)
+/// before the extension. Values that round to the same name would overwrite each other, so that is an error.
+fn frame_paths(out: &str, ts: &[f64]) -> Result<Vec<String>, String> {
+    if ts.len() == 1 {
+        return Ok(vec![out.to_string()]);
+    }
+    let p = std::path::Path::new(out);
+    let stem = p.file_stem().and_then(|x| x.to_str()).unwrap_or("frame");
+    let mut paths: Vec<String> = Vec::with_capacity(ts.len());
+    for t in ts {
+        let path = p.with_file_name(format!("{stem}-t{t:.3}.png")).to_string_lossy().into_owned();
+        if paths.contains(&path) {
+            return Err(format!("--t values round to the same three decimals: more than one frame would be written to {path}"));
+        }
+        paths.push(path);
+    }
+    Ok(paths)
+}
+
 fn convert(args: &[String]) -> Result<(), String> {
     let (Some(input), Some(out)) = (args.first(), args.get(1)) else { return Err("convert needs IN and OUT".into()) };
     let mut s = Session::new();
@@ -355,3 +368,25 @@ fn mcp(args: &[String]) -> Result<(), String> {
     let stdin = std::io::stdin();
     server.serve(stdin.lock(), std::io::stdout()).map_err(|e| e.to_string())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::frame_paths;
+
+    #[test]
+    fn single_frame_uses_out_as_is() {
+        assert_eq!(frame_paths("a/b.png", &[0.5]), Ok(vec!["a/b.png".to_string()]));
+    }
+
+    #[test]
+    fn frames_get_distinct_names() {
+        let paths = frame_paths("f.png", &[0.0, 0.25, 1.0]).unwrap_or_default();
+        assert_eq!(paths, ["f-t0.000.png", "f-t0.250.png", "f-t1.000.png"]);
+    }
+
+    #[test]
+    fn values_rounding_to_one_name_are_rejected() {
+        assert!(frame_paths("f.png", &[0.1001, 0.1002]).is_err());
+        assert!(frame_paths("f.png", &[0.5, 0.5]).is_err());
+    }
+}