diff --git a/src/core/main.js b/src/core/main.js
index a5c9a6c93d..5717cfec6b 100644
--- a/src/core/main.js
+++ b/src/core/main.js
@@ -125,7 +125,12 @@ class p5 {
     // If the user has created a global setup or draw function,
     // assume "global" mode and make everything global (i.e. on the window)
     if (!sketch) {
+      if (typeof window.p5 !== 'undefined') {
+        console.warn('p5.js appears to have been imported multiple times. This could cause errors.');
+        return;
+      }
       this._isGlobal = true;
+      window._p5Instance = this;
       if (window.hitCriticalError) {
         return;
       }