diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index 460ed549be92..31113feb1398 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -758,6 +758,8 @@ def __init__(
         self.pch: T.Dict[str, T.List[str]] = {}
         self.extra_args: T.DefaultDict[str, T.List[str]] = kwargs.get('language_args', defaultdict(list))
         self.sources: T.List[File] = []
+        # If the same source is defined multiple times, use it only once.
+        self.seen_sources: T.Set[File] = set()
         self.generated: T.List['GeneratedTypes'] = []
         self.extra_files: T.List[File] = []
         self.d_features: DFeatures = {
@@ -880,12 +882,11 @@ def process_sourcelist(self, sources: T.List['SourceOutputs']) -> None:
            (static as they are only regenerated if meson itself is regenerated)
         3. Sources files generated by another target or a Generator (generated)
         """
-        added_sources: T.Set[File] = set() # If the same source is defined multiple times, use it only once.
         for s in sources:
             if isinstance(s, File):
-                if s not in added_sources:
+                if s not in self.seen_sources:
                     self.sources.append(s)
-                    added_sources.add(s)
+                    self.seen_sources.add(s)
             elif isinstance(s, (CustomTarget, CustomTargetIndex, GeneratedList)):
                 self.generated.append(s)