1 files changed, 9 insertions, 2 deletions
diff --git a/src/resolver.rs b/src/resolver.rs
index 8d3a16d..d02d80f 100644
--- a/src/resolver.rs
+++ b/src/resolver.rs
@@ -114,8 +114,15 @@ impl Resolver {
     }
 
     /// Add a set of source units that might contain definitions for unresolved symbols.
-    pub fn add_library_source_units(&mut self, mut source_units: Vec<SourceUnit>) {
-        self.library_source_units.append(&mut source_units);
+    pub fn add_library_source_units(&mut self, source_units: Vec<SourceUnit>) {
+        for source_unit in source_units {
+            // Discard source units that have already been included.
+            let in_included = self.source_units.iter().any(|s| s.source_unit == source_unit);
+            let in_library = self.library_source_units.iter().any(|s| *s == source_unit);
+            if !in_included && !in_library {
+                self.library_source_units.push(source_unit);
+            }
+        }
     }
 
     /// Attempt to resolve unresolved references with library source units.