diff --git a/src/org/jruby/RubyModule.java b/src/org/jruby/RubyModule.java
index 10789b6..4da1b6c 100644
--- a/src/org/jruby/RubyModule.java
+++ b/src/org/jruby/RubyModule.java
@@ -2465,14 +2465,24 @@ public class RubyModule extends RubyObject {
     public IRubyObject getConstantAt(String name) {
         IRubyObject value = fetchConstant(name);
         
-        return value == UNDEF ? resolveUndefConstant(getRuntime(), name) : value;
+        if (value == UNDEF) {
+            resolveUndefConstant(getRuntime(), name);
+            value = fetchConstant(name);
+        }
+
+        return value;
     }
 
     public IRubyObject fastGetConstantAt(String internedName) {
         assert internedName == internedName.intern() : internedName + " is not interned";
         IRubyObject value = fastFetchConstant(internedName);
 
-        return value == UNDEF ? resolveUndefConstant(getRuntime(), internedName) : value;
+        if (value == UNDEF) {
+            resolveUndefConstant(getRuntime(), internedName);
+            value = fastFetchConstant(internedName);
+        }
+
+        return value;
     }
 
     /**
@@ -2516,8 +2526,10 @@ public class RubyModule extends RubyObject {
     protected IRubyObject getConstantInner(String name) {
         IRubyObject value = constantTableFetch(name);
 
-        for (; value == UNDEF; value = constantTableFetch(name)) {
-            if (resolveUndefConstant(getRuntime(), name) == null) return UNDEF;
+        if (value == UNDEF) {
+            resolveUndefConstant(getRuntime(), name);
+            value = constantTableFetch(name);
+            if (value == null) return UNDEF;
         }
         
         return value;
@@ -2540,7 +2552,7 @@ public class RubyModule extends RubyObject {
         while (p != null) {
             if ((value = p.constantTableFastFetch(internedName)) != null) {
                 if (value == UNDEF) {
-                    if (p.resolveUndefConstant(runtime, internedName) == null) break;
+                    p.resolveUndefConstant(runtime, internedName);
                     continue; // Not that is loaded loop around to resolve it next pass
                 }
 
@@ -2559,10 +2571,18 @@ public class RubyModule extends RubyObject {
                 "const_missing", getRuntime().fastNewSymbol(internedName));
     }
     
-    public IRubyObject resolveUndefConstant(Ruby runtime, String name) {
-        deleteConstant(name);
-        
-        return runtime.getLoadService().autoload(getName() + "::" + name);
+    public void resolveUndefConstant(Ruby runtime, String name) {
+        synchronized (runtime.getLoadService()) {
+            // We synchronize globally here to ensure only one thread can attempt
+            // autoload at a time; subsequent threads will see that the constant
+            // is no longer there or no longer UNDEF and not do the autoload,
+            // but all will at least wait until the first autoload has completed.
+            // See JRUBY-3194.
+            if (fetchConstant(name) == UNDEF) {
+                deleteConstant(name);
+                runtime.getLoadService().autoload(getName() + "::" + name);
+            }
+        }
     }
     
     /**
diff --git a/src/org/jruby/parser/StaticScope.java b/src/org/jruby/parser/StaticScope.java
index ab59023..5e8ebf7 100644
--- a/src/org/jruby/parser/StaticScope.java
+++ b/src/org/jruby/parser/StaticScope.java
@@ -154,7 +154,7 @@ public abstract class StaticScope implements Serializable {
 
     /* Try and unload the autoload specified from internedName */
     private IRubyObject getUndefConstant(Ruby runtime, String internedName, RubyModule object) {
-        if (cref.resolveUndefConstant(runtime, internedName) == null) return null;
+        cref.resolveUndefConstant(runtime, internedName);
 
         return getConstantInner(runtime, internedName, object);
     }