diff --git a/History.md b/History.md
index 26aba420e8..28d29632cc 100644
--- a/History.md
+++ b/History.md
@@ -14,6 +14,10 @@
 * Plugins should not process files whose names match the extension exactly (with
   no extra dot).  #3985
 
+* Adding the same file twice in the same package is now an error. Previously,
+  this could either lead to the file being included multiple times, or to a
+  build time crash.
+
 
 ## `meteor` command-line tool
 
diff --git a/tools/package-api.js b/tools/package-api.js
index eb849b7e77..35875107f5 100644
--- a/tools/package-api.js
+++ b/tools/package-api.js
@@ -77,7 +77,7 @@ function PackageAPI (options) {
 
   self.buildingIsopackets = !!options.buildingIsopackets;
 
-  // source files used
+  // source files used.  Map arch -> relPath -> {relPath, fileOptions}
   self.sources = {};
 
   // symbols exported
@@ -90,7 +90,7 @@ function PackageAPI (options) {
   self.implies = {};
 
   _.each(compiler.ALL_ARCHES, function (arch) {
-    self.sources[arch] = [];
+    self.sources[arch] = {};
     self.exports[arch] = [];
     self.uses[arch] = [];
     self.implies[arch] = [];
@@ -301,14 +301,26 @@ _.extend(PackageAPI.prototype, {
       return files.convertToPosixPath(p, true);
     });
 
+    var errors = [];
     _.each(paths, function (path) {
       forAllMatchingArchs(arch, function (a) {
+        if (_.has(self.sources[a], path)) {
+          errors.push("Duplicate source file: " + path);
+          return;
+        }
         var source = {relPath: path};
         if (fileOptions)
           source.fileOptions = fileOptions;
-        self.sources[a].push(source);
+        self.sources[a][path] = source;
       });
     });
+
+    // Spit out all the errors at the end, where the number of stack frames to
+    // skip is just 1 instead of something like 7 from forAllMatchingArchs and
+    // _.each.  Avoid using _.each here to keep stack predictable.
+    for (var i = 0; i < errors.length; ++i) {
+      buildmessage.error(errors[i], { useMyCaller: true });
+    }
   },
 
   // Use this release to resolve unclear dependencies for this package. If
diff --git a/tools/package-source.js b/tools/package-source.js
index 95f6a05b31..752e5d7489 100644
--- a/tools/package-source.js
+++ b/tools/package-source.js
@@ -974,7 +974,7 @@ _.extend(PackageSource.prototype, {
         // and then continue.
         api.sources = {};
         _.each(compiler.ALL_ARCHES, function (arch) {
-          api.sources[arch] = [];
+          api.sources[arch] = {};
         });
 
         fileAndDepLoader = null;
@@ -1103,7 +1103,7 @@ _.extend(PackageSource.prototype, {
         arch: arch,
         uses: api.uses[arch],
         implies: api.implies[arch],
-        getSourcesFunc: function () { return api.sources[arch]; },
+        getSourcesFunc: function () { return _.values(api.sources[arch]); },
         declaredExports: api.exports[arch],
         watchSet: watchSet
       }));