WSO2 Enterprise Integrator iterate and aggregate, issue with aggregation

I want to perform parallel API calls in iterate mediator then to group the responses using the aggregate mediator. The format is JSON. I am using this code:

 [...code in iterate]
<script language="js">var c = mc.getProperty("account");
                                  
                                    print("Value : "+ c );
                                    mc.setProperty("concatValue", c);
                                    mc.setPayloadJSON({"result":{"account" : c}});</script>
                                    
<log>
                 <property expression="json-eval($)" name="JSON-Payload in sequence"/>
              </log>
              

             </sequence>
       

        </target>
     </iterate>
		
	<property name="Aggregated_Responses" scope="default" value = "{}"/>
		
    <aggregate id="it1">
        <completeCondition>
          <messageCount max="-1" min="-1"/>
        </completeCondition>
        <onComplete expression="json-eval($)" enclosingElementProperty="Aggregated_Responses">
             <log>
                 <property expression="json-eval($)" name="JSON-Payload in oncomplete"/>
              </log>
        </onComplete>
   </aggregate>
  <loopback/>
</sequence>

The aggregation is not working, if i add loopback isnide the on complete, it will return one json and the second one will print a message in the logs saying that the response was already sent. Can someone help me solve this issue?
And an additional question: do the work inside the iterate is usually done in parallel or in series?

If you are using the Iterate Mediator you should have a Call Mediator within the iterate flow which does an external call. If not you have to use the Foreach Mediator instead.

Iterate Example

<?xml version="1.0" encoding="UTF-8"?>
<sequence name="iterate-aggregate-example" trace="disable" xmlns="http://ws.apache.org/ns/synapse">
  <property description="countRequests" expression="count(//Account)"
    name="countRequests" scope="default" type="STRING"/>
  <iterate expression="//Account" id="IterateAccountCreate"
sequential="true">
    <target>
      <sequence>
        <payloadFactory media-type="xml">
          <format>
            <echo:echoString xmlns_echo="http://echo.services.core.carbon.wso2.org">
              <in xmlns="">$1</in>
            </echo:echoString>
          </format>
          <args>
            <arg evaluator="xml" expression="//Account/text()"/>
          </args>
        </payloadFactory>
        <property action="remove" description="TRANSPORT_HEADERS"
          name="TRANSPORT_HEADERS" scope="axis2"/>
        <property description="OperationName" name="OperationName"
          scope="default" type="STRING" value="echoString"/>
        <header action="remove" name="To" scope="default"/>
        <header name="Action" scope="default" value="urn:echoString"/>
        <header name="SOAPAction" scope="transport" value="urn:echoString"/>
        <log category="DEBUG" description="***Request***">
          <property expression="$body" name="request"/>
        </log>
        <call>
          <endpoint key="echo"/>
        </call>
        <log category="DEBUG" description="***Response***" separator="***Response***">
          <property expression="$body" name="create_instanties_response"/>
        </log>
      </sequence>
    </target>
  </iterate>
  <aggregate description="" id="IterateAccountCreate">
    <completeCondition timeout="10">
      <messageCount max="{get-property('countRequests')}" min="{get
property('countRequests')}"/>
    </completeCondition>
    <onComplete expression="//ns:echoStringResponse" xmlns_ns="http://echo.services.core.carbon.wso2.org">
      <log description="***aggregated***" separator="***aggregated***">
        <property expression="$body" name="aggregated_body"/>      </log>
    </onComplete>
  </aggregate>
</sequence>

Final working solution with some explanation:

<payloadFactory media-type="json">
<format>
{               
"payloadss": [{
"account":"99999999999"
},{
"account":"6666666666"
}]
}
</format>
<args/>
</payloadFactory>
<iterate attachPath="json-eval($.payloadss)" expression="json-eval($.payloadss)" 
id="iterate-over" preservePayload="true" continueParent="false">
<target>
<sequence>
<property expression="json-eval($.payloadss.account)" name="accounte" scope="default" type="STRING"/>
<call-template target="call_template_a">
<with-param name="url" value="/getaccount"/>
<with-param name="method_type" value="get"/>
</call-template>
[.. apply error check etc .. ] 
</sequence>
</target>
</iterate>
[ use same id as the iterate otherwise it wont work ]
<property name="Aggregated_Responses" scope="default">
<jsonObject/>
</property>
<aggregate description="iterate-over" id="iterate-over">
<completeCondition timeout="120">
<messageCount max="-1" min="-1"/>
</completeCondition>
[ use expression like this otherwise it wont work, you can even add at the end / 
your field name if needed ]
<onComplete xmlns:ns="http://org.apache.synapse/xsd" expression="$body/*[1]" 
enclosingElementProperty="Aggregated_Responses">
[ here the json will be as payload, optionnally you can have a script mediator 
to map the value and change in their names, otherwise you can just return the 
result and it
will be aggregated together]
<property expression="json-eval($)" name="bbbb"/>
<script language="js"><![CDATA[
var dataFull = JSON.parse(mc.getProperty('bbbb')); 
var responseList = [];
var newJson = JSON.parse("{}");
for (i = 0; i < dataFull.length; i++) {
item = dataFull[i];
newJson.newAccountNumber = item.accountNumber;
responseList.push(newJson);
}
mc.setProperty("responseList", JSON.stringify(responseList));
]]></script>
<payloadFactory media-type="json">
<format>
{
"data": $2
}
</format>
<args>
<arg evaluator="xml" expression="get-property('responseList')"/>
</args>
</payloadFactory>
[ the send should be at this level, the code just after will not be executed] 
<send/>
</onComplete>

</aggregate>
</sequence>