Fix concurrency issue for Single definitions

koin-projects/koin-core/src/main/kotlin/org/koin/core/instance/SingleInstanceFactory.kt

@@ -44,8 +44,10 @@ class SingleInstanceFactory<T>(koin: Koin, beanDefinition: BeanDefinition<T>) :
 
     @Suppress("UNCHECKED_CAST")
     override fun get(context: InstanceContext): T {
-        if (!isCreated()) {
-            value = create(context)
+        synchronized(this) {
+            if (!isCreated()) {
+                value = create(context)
+            }
         }
         return value ?: error("Single instance created couldn't return value")
     }

koin-projects/koin-core/src/test/java/org/koin/core/SingleConcurrencyTest.kt

@@ -0,0 +1,43 @@
+package org.koin.core
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.koin.Simple
+import org.koin.dsl.koinApplication
+import org.koin.dsl.module
+import java.util.concurrent.Callable
+import java.util.concurrent.Executors
+
+class SingleConcurrencyTest {
+
+    @Test
+    fun `never creates two instances from different threads`() {
+        val executor = Executors.newFixedThreadPool(8)
+        // This test is repeated many times, because it only fails a very small % of times
+        repeat(3000) { repetition ->
+            val app = koinApplication {
+                modules(module {
+                    single { createComponent() }
+                })
+            }
+
+            val numberOfInstances = (1..50)
+                    .map { executor.submit(Callable { app.koin.get<Simple.ComponentA>() }) }
+                    .map { it.get() }
+                    .distinctBy { it.toString() }
+                    .count()
+
+
+            assertEquals(
+                    "More than one instance created concurrently for a `single` definition. Failed in repetition $repetition of the test.",
+                    1,
+                    numberOfInstances,
+            )
+        }
+    }
+
+    private fun createComponent(): Simple.ComponentA {
+        Thread.sleep(1) // Simulates a more expensive instance creation
+        return Simple.ComponentA()
+    }
+}