idocs totally processed

Hi everyone:

I have a BPM sending idocs to R/3, each execution into XI send an xml with lots of idocs, Those idocs are being executed through a workflow, the problem is that, probably if the workflow of the first idoc takes long time to finish the exectution the second idoc start the execution before the first finishes.

How can I solve this? in order to ensure that until the first idoc doesn´t finish completely the exectuion the second does not start to process.

Thanks.