Check if the archive has already been added to avoid duplicates

This avoids adding archives multiple times, which results in duplicate
objects in the resulting rlib, leading to symbol collision and link
failures. This could happen when crate contains multiple link attributes
that all reference the same archive.

src/librustc_codegen_llvm/back/archive.rs

@@ -36,11 +36,20 @@ enum Addition {
         name_in_archive: String,
     },
     Archive {
+        path: PathBuf,
         archive: ArchiveRO,
         skip: Box<dyn FnMut(&str) -> bool>,
     },
 }
 
+impl Addition {
+    fn path(&self) -> &Path {
+        match self {
+            Addition::File { path, .. } | Addition::Archive { path, .. } => path,
+        }
+    }
+}
+
 fn is_relevant_child(c: &Child<'_>) -> bool {
     match c.name() {
         Some(name) => !name.contains("SYMDEF"),
@@ -188,12 +197,16 @@ impl<'a> LlvmArchiveBuilder<'a> {
                       -> io::Result<()>
         where F: FnMut(&str) -> bool + 'static
     {
-        let archive = match ArchiveRO::open(archive) {
+        let archive_ro = match ArchiveRO::open(archive) {
             Ok(ar) => ar,
             Err(e) => return Err(io::Error::new(io::ErrorKind::Other, e)),
         };
+        if self.additions.iter().any(|ar| ar.path() == archive) {
+            return Ok(())
+        }
         self.additions.push(Addition::Archive {
-            archive,
+            path: archive.to_path_buf(),
+            archive: archive_ro,
             skip: Box::new(skip),
         });
         Ok(())
@@ -243,7 +256,7 @@ impl<'a> LlvmArchiveBuilder<'a> {
                         strings.push(path);
                         strings.push(name);
                     }
-                    Addition::Archive { archive, skip } => {
+                    Addition::Archive { archive, skip, .. } => {
                         for child in archive.iter() {
                             let child = child.map_err(string_to_io_error)?;
                             if !is_relevant_child(&child) {