check whether the subscriber exist before creating

components/apimgt-extensions/org.wso2.carbon.apimgt.application.extension.api/pom.xml

@@ -77,7 +77,18 @@
             <artifactId>commons-httpclient</artifactId>
             <scope>provided</scope>
         </dependency>
-
+        <dependency>
+            <groupId>org.wso2.carbon</groupId>
+            <artifactId>org.wso2.carbon.base</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>com.googlecode.json-simple.wso2</groupId>
+            <artifactId>json-simple</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.wso2.carbon</groupId>
+            <artifactId>org.wso2.carbon.user.api</artifactId>
+        </dependency>
         <dependency>
             <groupId>org.wso2.carbon</groupId>
             <artifactId>org.wso2.carbon.utils</artifactId>

components/apimgt-extensions/org.wso2.carbon.apimgt.application.extension/src/main/java/org/wso2/carbon/apimgt/application/extension/APIManagementProviderServiceImpl.java

@@ -286,14 +286,18 @@ public class APIManagementProviderServiceImpl implements APIManagementProviderSe
         try {
             APIConsumer consumer = APIManagerFactory.getInstance().getAPIConsumer(subscriberName);
             if (consumer != null) {
+                synchronized (consumer) {
+                    if (consumer.getSubscriber(subscriberName) == null) {
                         Subscriber subscriber = new Subscriber(subscriberName);
                         subscriber.setSubscribedDate(new Date());
                         subscriber.setEmail(subscriberEmail);
                         subscriber.setTenantId(tenantId);
                         consumer.addSubscriber(subscriber, groupId);
                         if (log.isDebugEnabled()) {
-                    log.debug("Successfully created subscriber with name : " + subscriberName + " with groupID : " +
+                            log.debug("Successfully created subscriber with name : " + subscriberName +
-                              groupId);
+                                              " with groupID : " + groupId);
+                        }
+                    }
                 }
             } else {
                 throw new APIManagerException("API provider configured for the given API configuration is null. " +