diff --git a/libraries/script-engine/src/ScriptEngine.cpp b/libraries/script-engine/src/ScriptEngine.cpp
index 0becf6a8dd..070bc98dbc 100644
--- a/libraries/script-engine/src/ScriptEngine.cpp
+++ b/libraries/script-engine/src/ScriptEngine.cpp
@@ -1188,20 +1188,15 @@ void ScriptEngine::include(const QStringList& includeFiles, QScriptValue callbac
             thisURL = resolvePath(file);
         }
 
-        if (!_includedURLs.contains(thisURL)) {
-            if (!isStandardLibrary && !currentSandboxURL.isEmpty() && (thisURL.scheme() == "file") &&
-                (currentSandboxURL.scheme() != "file" ||
-                 !thisURL.toString(strippingFlags).startsWith(currentSandboxURL.toString(strippingFlags), getSensitivity()))) {
-                qCWarning(scriptengine) << "Script.include() ignoring file path"
-                                        << thisURL << "outside of original entity script" << currentSandboxURL;
-            } else {
-                // We could also check here for CORS, but we don't yet.
-                // It turns out that QUrl.resolve will not change hosts and copy authority, so we don't need to check that here.
-                urls.append(thisURL);
-                _includedURLs << thisURL;
-            }
+        if (!isStandardLibrary && !currentSandboxURL.isEmpty() && (thisURL.scheme() == "file") &&
+            (currentSandboxURL.scheme() != "file" ||
+             !thisURL.toString(strippingFlags).startsWith(currentSandboxURL.toString(strippingFlags), getSensitivity()))) {
+            qCWarning(scriptengine) << "Script.include() ignoring file path"
+                                    << thisURL << "outside of original entity script" << currentSandboxURL;
         } else {
-            qCDebug(scriptengine) << "Script.include() ignoring previously included url:" << thisURL;
+            // We could also check here for CORS, but we don't yet.
+            // It turns out that QUrl.resolve will not change hosts and copy authority, so we don't need to check that here.
+            urls.append(thisURL);
         }
     }
 
@@ -1221,13 +1216,20 @@ void ScriptEngine::include(const QStringList& includeFiles, QScriptValue callbac
             if (contents.isNull()) {
                 qCDebug(scriptengine) << "Error loading file: " << url << "line:" << __LINE__;
             } else {
-                // Set the parent url so that path resolution will be relative
-                // to this script's url during its initial evaluation
-                _parentURL = url.toString();
-                auto operation = [&]() {
-                    evaluate(contents, url.toString());
-                };
-                doWithEnvironment(capturedEntityIdentifier, capturedSandboxURL, operation);
+                std::lock_guard<std::recursive_mutex> lock(_lock);
+                if (!_includedURLs.contains(url)) {
+                    _includedURLs << url;
+                    // Set the parent url so that path resolution will be relative
+                    // to this script's url during its initial evaluation
+                    _parentURL = url.toString();
+                    auto operation = [&]() {
+                        evaluate(contents, url.toString());
+                    };
+
+                    doWithEnvironment(capturedEntityIdentifier, capturedSandboxURL, operation);
+                } else {
+                    qCDebug(scriptengine) << "Script.include() skipping evaluation of previously included url:" << url;
+                }
             }
         }
         _parentURL = parentURL;
diff --git a/libraries/script-engine/src/ScriptEngine.h b/libraries/script-engine/src/ScriptEngine.h
index 01088660ff..2b2cb3c81a 100644
--- a/libraries/script-engine/src/ScriptEngine.h
+++ b/libraries/script-engine/src/ScriptEngine.h
@@ -245,6 +245,7 @@ protected:
 
     std::function<bool()> _emitScriptUpdates{ [](){ return true; }  };
 
+    std::recursive_mutex _lock;
 };
 
 #endif // hifi_ScriptEngine_h