diff --git a/src/header/unistd/mod.rs b/src/header/unistd/mod.rs
--- a/src/header/unistd/mod.rs
+++ b/src/header/unistd/mod.rs
@@ -273,7 +273,20 @@
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/dup.html>.
 // #[unsafe(no_mangle)]
 pub extern "C" fn dup3(fildes: c_int, fildes2: c_int, flag: c_int) -> c_int {
-    unimplemented!();
+    // dup3 requires fildes != fildes2 (unlike dup2 which is a no-op in that case)
+    if fildes == fildes2 {
+        ERRNO.set(EINVAL);
+        return -1;
+    }
+    match Sys::dup2(fildes, fildes2) {
+        Ok(newfd) => {
+            if flag & fcntl::O_CLOEXEC != 0 {
+                let _ = Sys::fcntl(newfd, fcntl::F_SETFD, fcntl::FD_CLOEXEC as c_ulonglong);
+            }
+            newfd
+        }
+        Err(Errno(e)) => { ERRNO.set(e); -1 }
+    }
 }
 
 // See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/encrypt.html>.