diff --git a/compiler/rustc_span/src/source_map.rs b/compiler/rustc_span/src/source_map.rs
index 81730f2f60869..562cf955e99a6 100644
--- a/compiler/rustc_span/src/source_map.rs
+++ b/compiler/rustc_span/src/source_map.rs
@@ -12,7 +12,7 @@
 pub use crate::hygiene::{ExpnData, ExpnKind};
 pub use crate::*;
 
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, StdEntry};
 use rustc_data_structures::stable_hasher::{Hash128, Hash64, StableHasher};
 use rustc_data_structures::sync::{IntoDynSyncSend, Lrc, MappedReadGuard, ReadGuard, RwLock};
 use std::cmp;
@@ -266,20 +266,24 @@ impl SourceMap {
         mut file: SourceFile,
     ) -> Result<Lrc<SourceFile>, OffsetOverflowError> {
         let mut files = self.files.borrow_mut();
+        let SourceMapFiles { ref mut source_files, ref mut stable_id_to_source_file } = &mut *files;
+        let file = match stable_id_to_source_file.entry(file_id) {
+            StdEntry::Occupied(o) => o.into_mut(),
+            StdEntry::Vacant(v) => {
+                file.start_pos = BytePos(if let Some(last_file) = source_files.last() {
+                    // Add one so there is some space between files. This lets us distinguish
+                    // positions in the `SourceMap`, even in the presence of zero-length files.
+                    last_file.end_position().0.checked_add(1).ok_or(OffsetOverflowError)?
+                } else {
+                    0
+                });
 
-        file.start_pos = BytePos(if let Some(last_file) = files.source_files.last() {
-            // Add one so there is some space between files. This lets us distinguish
-            // positions in the `SourceMap`, even in the presence of zero-length files.
-            last_file.end_position().0.checked_add(1).ok_or(OffsetOverflowError)?
-        } else {
-            0
-        });
-
-        let file = Lrc::new(file);
-        files.source_files.push(file.clone());
-        files.stable_id_to_source_file.insert(file_id, file.clone());
-
-        Ok(file)
+                let file = Lrc::new(file);
+                source_files.push(file.clone());
+                v.insert(file)
+            }
+        };
+        Ok(file.clone())
     }
 
     /// Creates a new `SourceFile`.